test: golden-file test for the plugin's generated code #8

Part of #2. A golden-file test for the plugin's output, the counterpart of Go's cmd/protoc-gen-go-mcp/golden_test.go at 9e072f9.

Do

• In cargo test, build a CodeGeneratorRequest for examples/protos/example.proto with source info (comments). Use protox, a pure-Rust protobuf compiler and the counterpart of Go's protocompile, so the test needs neither protoc nor the network.
  • Set compiler_version to 29.3 so the header matches make generate's output.
• Run generate on it, and compare the produced examples.v1.mcp.rs byte for byte with the committed examples/src/gen/examples.v1.mcp.rs.
• UPDATE_GOLDEN=1 cargo test -p protoc-gen-rust-mcp golden rewrites the golden file. Document this in AGENTS.md, replacing the "once the golden-file test exists" placeholder: an unexpected change in generated code is a bug until explained.
• On a mismatch, the failure message shows a readable diff and the update command.

Acceptance criteria

• The test passes on main's output, and fails with a useful diff if you hand-edit the golden file (show this in the PR).
• AGENTS.md is updated.
• The checks in AGENTS.md pass.