feat: comment processing, description fallback and file header (port from Go) #4

Part of #2. Port the text helpers in Go's cmd/protoc-gen-go-mcp/mcp.go at 9e072f9, with their tests from mcp_test.go. Tool descriptions and schema descriptions both depend on them, so their output must match Go's character for character.

Do

• Comments. Look up leading comments from SourceCodeInfo by path: the method, the message, the field, the enum, and the syntax/package detached comments for the file header.
• process_comment_to_string: a port of processCommentToString.
  • Handle line comments (strip // from continuation lines) and block comments (strip /* and */).
  • Join the lines with spaces, trim, and collapse whitespace runs into one space.
  • Do not escape anything; callers escape for their own context. This was a real bug in Go (#100): escaping here and again in the caller produced double escaping.
• camel_to_space: a port of camelToSpace, the fallback description when a method has no comment (e.g. SetVibeDetails → Set Vibe Details). Include the letter↔digit boundaries.
• File header: // @generated by protoc-gen-rust-mcp. DO NOT EDIT., followed by // versions: with the plugin version and protoc vX.Y.Z[-suffix] (or (unknown)) from compiler_version. This mirrors Go's protocVersion.
• Rust string literals: a helper that emits arbitrary text, such as descriptions and the JSON schema, as a valid Rust literal. Prefer a raw string r#"…"# with enough #s; it must survive any comment text, including "#, backslashes and backticks.

Acceptance criteria

• Port every case in Go's TestProcessCommentToString and TestUnexport/TestQuoteBacktickString (or their Rust-literal equivalents), plus the example proto's tricky comments: the block comment on SetVibe (quotes, a backtick, C:\vibes\new) and the multi-line GetVibe comment with its trailing empty // line. Each expected string must equal Go's output; take the expected values from the descriptions in Go's committed examples/gen/example/v1/example_mcp.pb.go.
• Add a property test or table test showing that the literal helper round-trips any string through syn::parse_str::<syn::LitStr>, or something equivalent.
• The checks in AGENTS.md pass.