Add a Cargo.toml lockfile parser (Rust)

[AshwinUgale]

Why

DocChat routes retrieval off the concrete versions a project pins. Today it reads Node (package.json/package-lock.json) and Python (pyproject.toml, requirements.txt) manifests. The README roadmap calls out Rust/Go/Ruby lockfiles as the next formats. This adds Rust.

What to do

• Add parse_cargo_toml(path) in sidecar/src/docchat_sidecar/lockfiles.py, following the exact shape of parse_pyproject_toml (returns list[Pin], empty list on missing/malformed, ValueError if the filename is wrong).
• Read [dependencies] and [dev-dependencies] from Cargo.toml. Handle both the string form serde = "1.0.188" and the table form serde = { version = "1.0.188", features = [...] }. Prefer the exact version from a sibling Cargo.lock when present (same "lock wins over range" rule the module docstring describes for npm).
• Export it in __all__ and reuse _strip_range for range strings.

Acceptance criteria

• New tests in sidecar/tests/test_lockfiles.py mirroring the existing pyproject cases: string dep, table dep, Cargo.lock exact-version win, missing file -> [], wrong filename -> ValueError.
• uv run ruff check . && uv run ruff format --check . && uv run mypy src && uv run pytest all green.

Pointers

• lockfiles.py: Pin dataclass, parse_pyproject_toml, _strip_range, __all__. tomllib is already imported.

New contributor? See CONTRIBUTING.md. Happy to help scope in the comments.