test: support dual-server E2E integration tests and protocol negotiation fallbacks

[anubhav756]

Overview

This PR updates the E2E testing infrastructure to support running integration tests against multiple parallel server configurations, specifically:

1. A standard Toolbox server (port 5000).
2. A draft-enabled Toolbox server (port 5001) run with the --enable-draft-specs flag.

By verifying the SDK's behavior against both environments, we ensure robust fallback and negotiation behavior when newer protocol drafts are configured.

Changes

1. E2E Test Infrastructure & Multi-Server Orchestration

• Updated to start and monitor two separate instances of the Toolbox server process:
  • ToolboxServer1 on port 5000 (stable).
  • ToolboxServer2 on port 5001 (with --enable-draft-specs).
• Introduced a helper killServer to gracefully terminate both server processes and clean up assets.
• Added support for a second process reference.

2. E2E Test Suites

• Refactored E2E tests in both @toolbox-sdk/core and @toolbox-sdk/adk to run all test cases against both servers (localhost:5000 and localhost:5001).
• Added Protocol Configuration E2E Tests which:
  • Check that the client defaults to the correct protocol version if none is specified.
  • Verify that the client successfully negotiates and falls back to an older supported protocol (MCP_v20251125) if the target server does not support the client's configured draft protocol (MCP_DRAFT_2026_v1).

3. Protocol & Client Updates

• Removed the verbose warning in ToolboxClient's constructor that alerted when initializing with a version other than MCP_LATEST.
  • This prevents spam when fallback / auto-negotiation takes place.

Note

This PR updates Protocol.MCP_LATEST to point to Protocol.MCP_DRAFT_2026_v1. This will be updated in the next PR down the chain (#394).

4. GCS Download Path Fix

• Fixed GCS download paths by removing the v prefix from the version directory, resolving download failures during local test runs.

5. Unit Test Adjustments

• Equipped MockTransport in client/tool unit tests with a mock protocolVersion property to satisfy current transport interface requirements.
• Removed obsolete assertions in unit tests checking for the deleted MCP_LATEST console warnings.

Note

This PR mirrors Python SDK changes from googleapis/mcp-toolbox-sdk-python#706.