feat(mcp): add comprehensive test coverage for MCP server (#455)

## Summary

Adds comprehensive test suite for MCP server components.

**Test coverage:**
- Server initialization and lifecycle
- JSON-RPC request/response handling
- All 6 tool implementations
- Edge cases and error paths

**Files added:**
- `mcp/server_test.go` - Server and handler tests
- `mcp/tools_test.go` - Tool implementation tests
- `mcp/types_test.go` - JSON serialization tests

Achieves baseline test coverage for the MCP module.

Author: shivasurya