refactor: move test fixtures out of the plugin's build.rs; format generated code #22

Part of #2. Follow-up from the reviews of #18 and #21.

Why

protoc-gen-rust-mcp/build.rs now always:

• compiles the test-only fixtures (tests/testdata/schemapb, tests/testdata/mcpgen) with protox, pbjson-build and tonic-prost-build
• runs the generator on them through #[path] includes, because a build script can't depend on the crate it's building

So every cargo build/cargo install of the plugin pulls in those build-dependencies and does fixture work. On a clean release build that took build time from about 3s to about 8s, before the tonic fixtures were added. Separately, the generated *.mcp.rs isn't formatted: there are 300-character lines, while tonic's output is run through prettyplease.

Do

• Move the fixtures and their build script into a non-published, dev-only workspace crate, e.g. test-fixtures/, with publish = false, that depends on the plugin crate as a normal library dependency. The plugin's own tests use it as a dev-dependency. The plugin crate should end up with no build.rs, or a trivial one.
• Run the generated MCP code through prettyplease (as protoc-gen-tonic does), or emit it already formatted. Regenerate, and update the golden file if test: golden-file test for the plugin's generated code #8 has landed.
• Keep every existing test and its coverage.

Acceptance criteria

• cargo tree -p protoc-gen-rust-mcp -e build,normal shows no protox, pbjson-build or tonic-prost-build.
• make generate output is rustfmt-clean: rustfmt --check --edition 2024 examples/src/gen/**/*.mcp.rs passes.
• The checks in AGENTS.md pass, and make generate and the golden file are current.