Production comments include linter appeasement and no-op symbol keeps

[nficano]

Most inline comments explain protocol decisions or test setup, but a few production comments and assignments are noise rather than useful explanation. src/arcp/_transport/stdio.py:64 binds loop = asyncio.get_running_loop() only to assign _ = loop at src/arcp/_transport/stdio.py:68, src/arcp/middleware/otel.py:128 imports and assigns TransportClosed only to silence an unused symbol, and src/arcp/_runtime/session.py:209 binds last_seen only to keep the symbol alive at src/arcp/_runtime/session.py:233. The repeated PLR0913 comments in src/arcp/_client/client.py:63 and src/arcp/_runtime/server.py:90 are more defensible because they explain an API-shape tradeoff, but the no-op symbol keeps make the code less idiomatic and train future contributors to preserve dead bindings.

Fix prompt: Remove unused imports, variables, and symbol-keep assignments instead of commenting around them. Keep comments that explain protocol behavior, compatibility tradeoffs, or concurrency invariants, and move broad lint rationale into configuration comments only when the exception applies to a whole API shape. After cleanup, run uv run ruff check ., uv run mypy src/arcp, and uv run pyright to confirm no suppressions were masking real type or lint issues.