mypy: clear gently/mesh/* type errors (Phase 4 of #49)

[subindevs]

Phase 4 of the incremental mypy effort (#49) — takes the gently/mesh/* cluster (15 errors / 3 modules) to clean and removes its entries from [[tool.mypy.overrides]].

Chose mesh as the first Phase-4 slice because it's the smallest self-contained cluster and low-risk. Deliberately skipped the denser gently/dataset/* cluster since #16 plans to retire that legacy SQLite code.

Changes

File	Errors	Fix
peer_client.py	13 × union-attr	Single root cause: self._session: ClientSession | None used after _ensure_session(). _ensure_session() now returns the non-None session; the 12 call sites use the returned local instead of self._session, so mypy can prove non-None. Behavior unchanged.
discovery.py	1 × override	_MeshProtocol.connection_made narrowed the param vs asyncio.BaseProtocol. Widened to BaseTransport (matches supertype) + assert isinstance(..., DatagramTransport) to narrow for the typed attribute.
capability_provider.py	1 × attr-defined	ctypes.windll is Windows-only in typeshed; targeted # type: ignore[attr-defined] on the already platform-guarded branch.
pyproject.toml	—	Removed the 3 mesh entries from the mypy override list.

Verification

• ✅ mypy . — no issues in 300 source files (deps-less, as CI runs)
• ✅ ruff check + ruff format --check — clean

Note: mesh unit tests were not run locally (environment lacks the py3.10–3.12 + uv + aiohttp deps). Changes are behavior-preserving by construction.

Part of #49.

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