test(mcp): add comprehensive tests for public MCP API access

- Add 15 tests covering all user stories in mcp_public_test.exs
- User Story 1: Package queries without authentication (6 tests)
  - search_packages, get_package, list_packages returns data without auth
  - invalid JSON-RPC request handling
  - non-existent package error handling
- User Story 2: Tool discovery (3 tests)
  - GET /mcp/tools returns tools without authentication
  - GET /mcp/server-info returns capabilities with auth status
  - All tools include inputSchema for client discovery
- User Story 3: Dependencies (3 tests)
  - get_package_metadata returns dependencies without auth
  - get_dependency_tree returns structure without auth
  - check_compatibility works without authentication
- User Story 4: Documentation (2 tests)
  - Package info includes external links without auth
  - get_documentation tool accessible without authentication
- Cross-cutting concerns (2 tests)
  - Rate limiting test with IP-based tracking
  - MCP disabled returns error -32001 with 503 status

Author: GSMLG-BOT