feat(spanner): add spanner-get-database-ddl tool

[anubhav756]

Description

This PR introduces the spanner-get-database-ddl tool, allowing MCP agents to retrieve the complete schema definition (DDL statements) of a Cloud Spanner database.

Changes:

• Added DatabaseAdminClient lifecycle management and lazy on-demand initialization to Spanner Source.
• Implemented GetDatabaseDdl(ctx, tokenString) on Spanner Source using the Cloud Spanner Database Admin API.
• Created spanner-get-database-ddl with ReadOnlyAnnotations (readOnlyHint: true, destructiveHint: false).
• Registered the tool in imports.go and added reference documentation in spanner-get-database-ddl.md.

[gemini-code-assist]

Code Review

This pull request introduces a new Spanner tool, spanner-get-database-ddl, which retrieves the DDL statements defining the schema of a Cloud Spanner database. This includes adding the tool implementation, updating the Spanner source to support fetching the database DDL via a Spanner admin client, adding unit tests, and providing documentation. The reviewer noted an issue in the tool's Invoke method where authentication errors from ParseBearerToken are incorrectly wrapped and returned as internal server errors (500) instead of unauthorized errors (401), and suggested propagating the original error.

[gemini-code-assist]

When accessToken.ParseBearerToken() fails, it already returns a util.ToolboxError with a 401 Unauthorized status code. Wrapping it in a new util.NewClientServerError with http.StatusInternalServerError overrides this and returns a 500 Internal Server Error to the client, which is misleading for authentication failures. We should propagate the original util.ToolboxError if present, or default to http.StatusUnauthorized.

	if source.UseClientAuthorization() {
		tokenStr, err = accessToken.ParseBearerToken()
		if err != nil {
			if toolboxErr, ok := err.(util.ToolboxError); ok {
				return nil, toolboxErr
			}
			return nil, util.NewClientServerError("failed to parse access token", http.StatusUnauthorized, err)
		}
	}