fix: format BigQuery numeric output as decimal strings (#2084)

## Description

> Should include a concise description of the changes (bug or feature),
it's
> impact, along with a summary of the solution

This change updates both `bigquery-sql` and `bigquery-execute-sql` tools
to format `NUMERIC` and `BIGNUMERIC` values as decimal strings (e.g.,
"9.5") instead of rational fractions (e.g., "19/2"). This ensures the
tools' output matches the BigQuery REST API JSON format.

Key changes:
- Added `NormalizeValue` function in
`internal/tools/bigquery/bigquerycommon` to handle `*big.Rat` conversion
with 38-digit precision and trailing zero trimming.
- Updated `bigquery-sql` and `bigquery-execute-sql` to use
`NormalizeValue`.
- Added comprehensive tests in
`internal/tools/bigquery/bigquerycommon/conversion_test.go`.

With these changes the formatting for NUMERIC and BIGNUMERIC is fixed.

**Before:**

```
[
  {
    "id": 3,
    "numeric_value": "1"
  },
  {
    "id": 2,
    "numeric_value": "333333333/1000000000"
  },
  {
    "id": 4,
    "numeric_value": "12341/10"
  },
  {
    "id": 1,
    "numeric_value": "19/2"
  }
]
```

**After:**

```
[
  {
    "id": 3,
    "numeric_value": "1"
  },
  {
    "id": 2,
    "numeric_value": "0.333333333"
  },
  {
    "id": 4,
    "numeric_value": "1234.1"
  },
  {
    "id": 1,
    "numeric_value": "9.5"
  }
]
```

## PR Checklist

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[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #1194

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Averi Kitsch <akitsch@google.com>

internal/tools/bigquery/bigquerycommon/conversion_test.go

@@ -0,0 +1,123 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package bigquerycommon
+
+import (
+	"math/big"
+	"reflect"
+	"testing"
+)
+
+func TestNormalizeValue(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    any
+		expected any
+	}{
+		{
+			name:     "big.Rat 1/3 (NUMERIC scale 9)",
+			input:    new(big.Rat).SetFrac64(1, 3),               // 0.33333333333...
+			expected: "0.33333333333333333333333333333333333333", // FloatString(38)
+		},
+		{
+			name:     "big.Rat 19/2 (9.5)",
+			input:    new(big.Rat).SetFrac64(19, 2),
+			expected: "9.5",
+		},
+		{
+			name:     "big.Rat 12341/10 (1234.1)",
+			input:    new(big.Rat).SetFrac64(12341, 10),
+			expected: "1234.1",
+		},
+		{
+			name:     "big.Rat 10/1 (10)",
+			input:    new(big.Rat).SetFrac64(10, 1),
+			expected: "10",
+		},
+		{
+			name:     "string",
+			input:    "hello",
+			expected: "hello",
+		},
+		{
+			name:     "int",
+			input:    123,
+			expected: 123,
+		},
+		{
+			name: "nested slice of big.Rat",
+			input: []any{
+				new(big.Rat).SetFrac64(19, 2),
+				new(big.Rat).SetFrac64(1, 4),
+			},
+			expected: []any{"9.5", "0.25"},
+		},
+		{
+			name: "nested map of big.Rat",
+			input: map[string]any{
+				"val1": new(big.Rat).SetFrac64(19, 2),
+				"val2": new(big.Rat).SetFrac64(1, 2),
+			},
+			expected: map[string]any{
+				"val1": "9.5",
+				"val2": "0.5",
+			},
+		},
+		{
+			name: "complex nested structure",
+			input: map[string]any{
+				"list": []any{
+					map[string]any{
+						"rat": new(big.Rat).SetFrac64(3, 2),
+					},
+				},
+			},
+			expected: map[string]any{
+				"list": []any{
+					map[string]any{
+						"rat": "1.5",
+					},
+				},
+			},
+		},
+		{
+			name: "slice of *big.Rat",
+			input: []*big.Rat{
+				new(big.Rat).SetFrac64(19, 2),
+				new(big.Rat).SetFrac64(1, 4),
+			},
+			expected: []any{"9.5", "0.25"},
+		},
+		{
+			name:     "slice of strings",
+			input:    []string{"a", "b"},
+			expected: []any{"a", "b"},
+		},
+		{
+			name:     "byte slice (BYTES)",
+			input:    []byte("hello"),
+			expected: []byte("hello"),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := NormalizeValue(tt.input)
+			if !reflect.DeepEqual(got, tt.expected) {
+				t.Errorf("NormalizeValue() = %v, want %v", got, tt.expected)
+			}
+		})
+	}
+}

internal/tools/bigquery/bigquerycommon/util.go

@@ -17,6 +17,8 @@ package bigquerycommon
 import (
 	"context"
 	"fmt"
+	"math/big"
+	"reflect"
 	"sort"
 	"strings"
 
@@ -118,3 +120,54 @@ func InitializeDatasetParameters(
 
 	return projectParam, datasetParam
 }
+
+// NormalizeValue converts BigQuery specific types to standard JSON-compatible types.
+// Specifically, it handles *big.Rat (used for NUMERIC/BIGNUMERIC) by converting
+// them to decimal strings with up to 38 digits of precision, trimming trailing zeros.
+// It recursively handles slices (arrays) and maps (structs) using reflection.
+func NormalizeValue(v any) any {
+	if v == nil {
+		return nil
+	}
+
+	// Handle *big.Rat specifically.
+	if rat, ok := v.(*big.Rat); ok {
+		// Convert big.Rat to a decimal string.
+		// Use a precision of 38 digits (enough for BIGNUMERIC and NUMERIC)
+		// and trim trailing zeros to match BigQuery's behavior.
+		s := rat.FloatString(38)
+		if strings.Contains(s, ".") {
+			s = strings.TrimRight(s, "0")
+			s = strings.TrimRight(s, ".")
+		}
+		return s
+	}
+
+	// Use reflection for slices and maps to handle various underlying types.
+	rv := reflect.ValueOf(v)
+	switch rv.Kind() {
+	case reflect.Slice, reflect.Array:
+		// Preserve []byte as is, so json.Marshal encodes it as Base64 string (BigQuery BYTES behavior).
+		if rv.Type().Elem().Kind() == reflect.Uint8 {
+			return v
+		}
+		newSlice := make([]any, rv.Len())
+		for i := 0; i < rv.Len(); i++ {
+			newSlice[i] = NormalizeValue(rv.Index(i).Interface())
+		}
+		return newSlice
+	case reflect.Map:
+		// Ensure keys are strings to produce a JSON-compatible map.
+		if rv.Type().Key().Kind() != reflect.String {
+			return v
+		}
+		newMap := make(map[string]any, rv.Len())
+		iter := rv.MapRange()
+		for iter.Next() {
+			newMap[iter.Key().String()] = NormalizeValue(iter.Value().Interface())
+		}
+		return newMap
+	}
+
+	return v
+}

internal/tools/bigquery/bigqueryexecutesql/bigqueryexecutesql.go

@@ -337,7 +337,7 @@ func (t Tool) Invoke(ctx context.Context, resourceMgr tools.SourceProvider, para
 		schema := it.Schema
 		row := orderedmap.Row{}
 		for i, field := range schema {
-			row.Add(field.Name, val[i])
+			row.Add(field.Name, bqutil.NormalizeValue(val[i]))
 		}
 		out = append(out, row)
 	}

internal/tools/bigquery/bigquerysql/bigquerysql.go

@@ -274,7 +274,7 @@ func (t Tool) Invoke(ctx context.Context, resourceMgr tools.SourceProvider, para
 		}
 		vMap := make(map[string]any)
 		for key, value := range row {
-			vMap[key] = value
+			vMap[key] = bqutil.NormalizeValue(value)
 		}
 		out = append(out, vMap)
 	}