feat(spanner): add spanner-update-database-schema tool

[anubhav756]

Description

This PR introduces the spanner-update-database-schema tool for executing DDL schema mutations (CREATE TABLE, ALTER TABLE, CREATE INDEX, etc.) on Cloud Spanner databases, along with automatic agent suppression on read-only sources.

Changes:

• Implemented UpdateDatabaseDdl(ctx, statements, tokenString) on Spanner Source using DatabaseAdminClient.UpdateDatabaseDdl and waiting for long-running operation completion via op.Wait(ctx).
• Created spanner-update-database-schema with destructive annotations (readOnlyHint: false, destructiveHint: true).
• Enabled automatic tool suppression via tools.ShouldSuppress so the tool is hidden from MCP agents when connected to a read-only Spanner source.
• Added update_database_schema and get_database_ddl to the data and data_with_discovery toolsets in spanner.yaml and spanner-postgres.yaml.
• Updated config_test.go and unit tests to validate schema updating and suppression behavior.

[gemini-code-assist]

Code Review

This pull request introduces the spanner-update-database-schema tool to execute DDL statements on Cloud Spanner databases, along with corresponding updates to prebuilt configurations, tests, and documentation. Feedback on the implementation highlights a potential issue where non-string or empty elements in the statements array are silently skipped, which could lead to partial migration execution; it is recommended to return an error instead.

[gemini-code-assist]

Silently skipping non-string or empty elements in the statements array can lead to partial execution of a DDL migration, which can leave the database schema in an inconsistent or broken state. It is safer to return an error if any element in the array is not a valid, non-empty string.

	case []any:
		for _, elem := range v {
			str, ok := elem.(string)
			if !ok {
				return nil, util.NewAgentError(fmt.Sprintf("invalid element type in 'statements': expected string, got %T", elem), nil)
			}
			if str == "" {
				return nil, util.NewAgentError("empty statement is not allowed", nil)
			}
			statements = append(statements, str)
		}