fix(parser): detect Rust #[test] attribute and emit Test-kind nodes

[TimERTy]

Summary

Closes #502.

Rust #[test] functions were indexed as kind="Function" with is_test=False. tests_for(...) returned 0 edges for any Rust target, detect_changes flagged every test fn as untested, and test-gap counts were inflated by one per real test.

Root cause

Three compounding gaps in code_review_graph/parser.py:

1. _extract_function doesn't read Rust attributes. Decorators are collected only for Java/Kotlin/C# (via modifiers child) and Python (via decorated_definition parent). In tree-sitter-rust, attributes are preceding siblings of function_item in the enclosing scope, not children — so decorators was always () for Rust.
2. _TEST_ANNOTATIONS missing Rust entries. Only JUnit annotations were listed.
3. Fixture had no #[test] fn. tests/fixtures/sample_rust.rs had zero #[test] functions and TestRustParsing never asserted on kind == "Test", so the bug shipped invisible to the suite.

Changes

• code_review_graph/parser.py
  • In _extract_function, walk attribute_item preceding siblings when language == "rust", strip #[ ] / #![ ] wrapping, and append the inner text to deco_list.
  • Extend _TEST_ANNOTATIONS with test, tokio::test, async_std::test, rstest, rstest::rstest, proptest.
• tests/fixtures/sample_rust.rs — add #[cfg(test)] mod tests { ... } with #[test] and #[tokio::test] functions.
• tests/test_multilang.py::TestRustParsing — three new assertions:
  • test_detects_test_attribute — #[test] fns produce kind="Test".
  • test_detects_tokio_test_attribute — #[tokio::test] likewise.
  • test_non_test_functions_not_misclassified — non-annotated fns stay kind="Function", guarding against over-broad attribute matching.

Test plan

• pytest tests/test_multilang.py::TestRustParsing -v — 8/8 pass (5 existing + 3 new).
• pytest tests/test_multilang.py tests/test_parser.py — 406 passed, no regressions.
• Verified on an external Rust crate (a small CLI with #[cfg(test)] mod tests in src/): before patch, 0 Test-kind nodes; after patch, every #[test] fn correctly tagged Test, every non-annotated fn (incl. a fn parse(...) helper inside the test module) stays Function.

Notes

• Derive macros like #[derive(Debug, Clone)] get added to the decorator list too — they don't match any entry in _TEST_ANNOTATIONS, so no false positives.
• #[cfg_attr(test, test)] and other conditional wrappers are out of scope; covered by the literal #[test] path that's overwhelmingly common.