feat(prebuilt/mysql): Add a new tool to show query plan of a given query in MySQL (#1474)

## Description

Adds a read-only MySQL prebuilt tool, `get_query_plan`, that returns the
optimizer’s estimated plan for a single SQL statement using `EXPLAIN
FORMAT JSON`.

## PR Checklist

---
> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [x] 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)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

---------

Co-authored-by: Averi Kitsch <akitsch@google.com>

cmd/root_test.go

@@ -1429,7 +1429,7 @@ func TestPrebuiltTools(t *testing.T) {
 			wantToolset: server.ToolsetConfigs{
 				"cloud_sql_mysql_database_tools": tools.ToolsetConfig{
 					Name:      "cloud_sql_mysql_database_tools",
-					ToolNames: []string{"execute_sql", "list_tables"},
+					ToolNames: []string{"execute_sql", "get_query_plan", "list_tables"},
 				},
 			},
 		},
@@ -1469,7 +1469,7 @@ func TestPrebuiltTools(t *testing.T) {
 			wantToolset: server.ToolsetConfigs{
 				"mysql_database_tools": tools.ToolsetConfig{
 					Name:      "mysql_database_tools",
-					ToolNames: []string{"execute_sql", "list_tables"},
+					ToolNames: []string{"execute_sql", "get_query_plan", "list_tables"},
 				},
 			},
 		},

docs/en/reference/prebuilt-tools.md

@@ -99,6 +99,7 @@ See guides, [Connect from your IDE](../how-to/connect-ide/_index.md), for detail
 *   **Tools:**
     *   `execute_sql`: Executes a SQL query.
     *   `list_tables`: Lists tables in the database.
+    *   `get_query_plan`: Provides information about how MySQL executes a SQL statement.
 
 ## Cloud SQL for MySQL Observability
 
@@ -255,6 +256,7 @@ See guides, [Connect from your IDE](../how-to/connect-ide/_index.md), for detail
 *   **Tools:**
     *   `execute_sql`: Executes a SQL query.
     *   `list_tables`: Lists tables in the database.
+    *   `get_query_plan`: Provides information about how MySQL executes a SQL statement.
 
 ## OceanBase
 

internal/prebuiltconfigs/tools/cloud-sql-mysql.yaml

@@ -27,6 +27,17 @@ tools:
     kind: mysql-execute-sql
     source: cloud-sql-mysql-source
     description: Use this tool to execute SQL.
+  get_query_plan:
+    kind: mysql-sql
+    source: cloud-sql-mysql-source
+    description: "Provide information about how MySQL executes a SQL statement. Common use cases include: 1) analyze query plan to improve its performance, and 2) determine effectiveness of existing indexes and evalueate new ones."
+    statement: |
+      EXPLAIN FORMAT=JSON {{.sql_statement}};
+    templateParameters:
+      - name: sql_statement
+        type: string
+        description: "the SQL statement to explain"
+        required: true
   list_tables:
     kind: mysql-list-tables
     source: cloud-sql-mysql-source
@@ -35,4 +46,5 @@ tools:
 toolsets:
   cloud_sql_mysql_database_tools:
     - execute_sql
+    - get_query_plan
     - list_tables

internal/prebuiltconfigs/tools/mysql.yaml

@@ -31,6 +31,17 @@ tools:
     kind: mysql-execute-sql
     source: mysql-source
     description: Use this tool to execute SQL.
+  get_query_plan:
+    kind: mysql-sql
+    source: mysql-source
+    description: "Provide information about how MySQL executes a SQL statement. Common use cases include: 1) analyze query plan to improve its performance, and 2) determine effectiveness of existing indexes and evalueate new ones."
+    statement: |
+      EXPLAIN FORMAT=JSON {{.sql_statement}};
+    templateParameters:
+      - name: sql_statement
+        type: string
+        description: "the SQL statement to explain"
+        required: true
   list_tables:
     kind: mysql-list-tables
     source: mysql-source
@@ -39,4 +50,5 @@ tools:
 toolsets:
   mysql_database_tools:
     - execute_sql
+    - get_query_plan
     - list_tables