test(source/alloydb): create MCP integration tests (#2843)

## Overview

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

## Changes
- Created `tests/alloydb/alloydb_mcp_test.go` to test database listing
and cluster operations over the native `/mcp` pathway.
- Achieves parity with coverage from the legacy
`alloydb_integration_test.go`.

## Checklist
- [x] Ensure the tests and linter pass

Author: anubhav756