mcp: refactor for testability and add comprehensive test suite

- Refactor mcp.Server and router to accept injected I/O streams.
- Implement McpHarness for high-fidelity MCP integration testing.
- Add unit tests for protocol, tools, and resources modules.
- Add integration tests covering initialization, tool/resource execution, and error handling.
- Improve error reporting for malformed JSON requests.

src/mcp.zig

@@ -1,3 +1,4 @@
 pub const Server = @import("mcp/Server.zig");
 pub const protocol = @import("mcp/protocol.zig");
 pub const router = @import("mcp/router.zig");
+pub const testing = @import("mcp/testing.zig");