fix: narrow send_cancel BaseException catch to Exception

[rasdani]

Summary

ZMQEnvClient.send_cancel caught BaseException and silently passed. Intent was best-effort cleanup — transport/socket errors shouldn't break shutdown. But BaseException also swallows:

• asyncio.CancelledError — denying the caller's cancellation. Particularly ironic in a method literally named send_cancel.
• KeyboardInterrupt — eating user Ctrl-C during cleanup.
• SystemExit — ignoring explicit sys.exit().

Narrowed to except Exception: pass. Transport / socket / ZMQ errors still swallowed as before; cancellation and interrupt signals now propagate correctly.

Changes

• `verifiers/serve/client/zmq_env_client.py` — single-line catch narrowing with comment
• `tests/test_env_server.py` — added `TestSendCancelErrorHandling` with 5 tests:
  • `test_send_cancel_swallows_connection_error` — regression: transport errors silently swallowed
  • `test_send_cancel_swallows_oserror` — regression: OSError silently swallowed
  • `test_send_cancel_propagates_cancelled_error` — the headline bug
  • `test_send_cancel_propagates_keyboard_interrupt`
  • `test_send_cancel_propagates_system_exit`

Verification

• `uv run pytest tests/test_env_server.py -x -q` — 15 passed (10 existing + 5 new)
• `uv run ruff check` + `ruff format --check` — clean

Context

Part of a small audit of `BaseException` catches in verifiers following bugbot's review on #1196. See the plan file for the full inventory; companion PR is #1197 (same narrowing applied to `math_verify` calls in `math_rubric.py`).

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Note

Low Risk
Low risk: a narrowly-scoped exception-handling tweak in send_cancel plus targeted tests; behavior only changes for cancellation/interrupt signals while keeping transport errors best-effort.

Overview
ZMQEnvClient.send_cancel now catches Exception instead of BaseException, so best-effort cancel notifications still ignore socket/transport failures but no longer suppress asyncio.CancelledError, KeyboardInterrupt, or SystemExit.

Adds a new TestSendCancelErrorHandling suite verifying transport errors are swallowed and cancellation/interrupt signals propagate.

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