refactor: decouple Source from Tool (#2204)

This PR update the linking mechanism between Source and Tool.

Tools are directly linked to their Source, either by pointing to the
Source's functions or by assigning values from the source during Tool's
initialization. However, the existing approach means that any
modification to the Source after Tool's initialization might not be
reflected. To address this limitation, each tool should only store a
name reference to the Source, rather than direct link or assigned
values.

Tools will provide interface for `compatibleSource`. This will be used
to determine if a Source is compatible with the Tool.
```
type compatibleSource interface{
    Client() http.Client
    ProjectID() string
}
```

During `Invoke()`, the tool will run the following operations:
* retrieve Source from the `resourceManager` with source's named defined
in Tool's config
* validate Source via `compatibleSource interface{}`
* run the remaining `Invoke()` function. Fields that are needed is
retrieved directly from the source.

With this update, resource manager is also added as input to other
Tool's function that require access to source (e.g.
`RequiresClientAuthorization()`).

internal/tools/cloudsqlmysql/cloudsqlmysqlcreateinstance/cloudsqlmysqlcreateinstance.go

@@ -21,7 +21,6 @@ import (
 
 	yaml "github.com/goccy/go-yaml"
 	"github.com/googleapis/genai-toolbox/internal/sources"
-	"github.com/googleapis/genai-toolbox/internal/sources/cloudsqladmin"
 	"github.com/googleapis/genai-toolbox/internal/tools"
 	"github.com/googleapis/genai-toolbox/internal/util/parameters"
 	sqladmin "google.golang.org/api/sqladmin/v1"
@@ -43,6 +42,12 @@ func newConfig(ctx context.Context, name string, decoder *yaml.Decoder) (tools.T
 	return actual, nil
 }
 
+type compatibleSource interface {
+	GetDefaultProject() string
+	GetService(context.Context, string) (*sqladmin.Service, error)
+	UseClientAuthorization() bool
+}
+
 // Config defines the configuration for the create-instances tool.
 type Config struct {
 	Name         string   `yaml:"name" validate:"required"`
@@ -66,12 +71,12 @@ func (cfg Config) Initialize(srcs map[string]sources.Source) (tools.Tool, error)
 	if !ok {
 		return nil, fmt.Errorf("no source named %q configured", cfg.Source)
 	}
-	s, ok := rawS.(*cloudsqladmin.Source)
+	s, ok := rawS.(compatibleSource)
 	if !ok {
 		return nil, fmt.Errorf("invalid source for %q tool: source kind must be `cloud-sql-admin`", kind)
 	}
 
-	project := s.DefaultProject
+	project := s.GetDefaultProject()
 	var projectParam parameters.Parameter
 	if project != "" {
 		projectParam = parameters.NewStringParameterWithDefault("project", project, "The GCP project ID. This is pre-configured; do not ask for it unless the user explicitly provides a different one.")
@@ -96,7 +101,6 @@ func (cfg Config) Initialize(srcs map[string]sources.Source) (tools.Tool, error)
 
 	return Tool{
 		Config:      cfg,
-		Source:      s,
 		AllParams:   allParameters,
 		manifest:    tools.Manifest{Description: cfg.Description, Parameters: paramManifest, AuthRequired: cfg.AuthRequired},
 		mcpManifest: mcpManifest,
@@ -106,7 +110,6 @@ func (cfg Config) Initialize(srcs map[string]sources.Source) (tools.Tool, error)
 // Tool represents the create-instances tool.
 type Tool struct {
 	Config
-	Source      *cloudsqladmin.Source
 	AllParams   parameters.Parameters `yaml:"allParams"`
 	manifest    tools.Manifest
 	mcpManifest tools.McpManifest
@@ -118,6 +121,11 @@ func (t Tool) ToConfig() tools.ToolConfig {
 
 // Invoke executes the tool's logic.
 func (t Tool) Invoke(ctx context.Context, resourceMgr tools.SourceProvider, params parameters.ParamValues, accessToken tools.AccessToken) (any, error) {
+	source, err := tools.GetCompatibleSource[compatibleSource](resourceMgr, t.Source, t.Name, t.Kind)
+	if err != nil {
+		return nil, err
+	}
+
 	paramsMap := params.AsMap()
 
 	project, ok := paramsMap["project"].(string)
@@ -167,7 +175,7 @@ func (t Tool) Invoke(ctx context.Context, resourceMgr tools.SourceProvider, para
 		Project:         project,
 	}
 
-	service, err := t.Source.GetService(ctx, string(accessToken))
+	service, err := source.GetService(ctx, string(accessToken))
 	if err != nil {
 		return nil, err
 	}
@@ -200,10 +208,14 @@ func (t Tool) Authorized(verifiedAuthServices []string) bool {
 	return true
 }
 
-func (t Tool) RequiresClientAuthorization(resourceMgr tools.SourceProvider) bool {
-	return t.Source.UseClientAuthorization()
+func (t Tool) RequiresClientAuthorization(resourceMgr tools.SourceProvider) (bool, error) {
+	source, err := tools.GetCompatibleSource[compatibleSource](resourceMgr, t.Source, t.Name, t.Kind)
+	if err != nil {
+		return false, err
+	}
+	return source.UseClientAuthorization(), nil
 }
 
-func (t Tool) GetAuthTokenHeaderName() string {
-	return "Authorization"
+func (t Tool) GetAuthTokenHeaderName(resourceMgr tools.SourceProvider) (string, error) {
+	return "Authorization", nil
 }