test(source/cloudsqlpg): create MCP integration tests

[anubhav756]

This PR adds the mapped integration tests for CloudSQL PG tools using the native MCP harness.

[gemini-code-assist]

Code Review

This pull request adds integration tests for the Cloud SQL Postgres data source, including tests for listing and calling tools. The review identifies several improvement opportunities: refactoring the connection pool setup to eliminate code duplication and use production logic, enhancing environment variable validation to report all missing variables simultaneously, and renaming validation functions to better reflect their purpose while ensuring consistent application across test cases.

[gemini-code-assist]

This switch "" pattern for checking environment variables is clever, but it has a downside: it fails the test on the first missing variable. This can be frustrating for developers who then have to fix one variable and rerun tests, only to find another one is missing.

A more developer-friendly approach is to check for all required variables and report all missing ones at once. This gives a complete picture of the required setup.

	vars := map[string]string{
		"CLOUD_SQL_POSTGRES_PROJECT":  CloudSQLPostgresProject,
		"CLOUD_SQL_POSTGRES_REGION":   CloudSQLPostgresRegion,
		"CLOUD_SQL_POSTGRES_INSTANCE": CloudSQLPostgresInstance,
		"CLOUD_SQL_POSTGRES_DATABASE": CloudSQLPostgresDatabase,
		"CLOUD_SQL_POSTGRES_USER":     CloudSQLPostgresUser,
		"CLOUD_SQL_POSTGRES_PASS":     CloudSQLPostgresPass,
	}
	var missing []string
	for key, val := range vars {
		if val == "" {
			missing = append(missing, key)
		}
	}
	if len(missing) > 0 {
		t.Fatalf("Missing required environment variables: %s", strings.Join(missing, ", "))
	}

[gemini-code-assist]

The getCloudSQLPgVars function is called here, but its return value is ignored. This is confusing because the function name suggests it gets something, but it's being used only for the side-effect of validating environment variables.

To improve clarity, consider renaming the function to requireCloudSQLPgVars to make its purpose as a validator explicit.

Also, TestCloudSQLPgListTools would benefit from this check at the beginning, as it also depends on these environment variables to start the server with the prebuilt configuration.