test: raise unit test coverage to 100% across all SDK modules

[zeevdr]

Summary

• Coverage had reached 97.52% from prior work but 23 statements across 6 modules remained uncovered; this PR closes every gap to reach 100%.
• The missing branches were edge-case paths: spurious async-event wakes, deadline-budget exhaustion in the async retry loop, stream-loop bodies that existing tests never exercised (all mocks used empty streams), early-return paths when a watcher is stopped mid-error, and the fallback branch in the TypedValue match.
• Hitting 100% removes the risk of undetected regressions in these exact paths, which include production-critical reconnect and error-handling logic.

Test plan

• _channel.py: capture and invoke the inner callback to confirm Bearer header injection
• _convert.py: fake TypedValue with unknown kind to exercise the fallback case
• _retry.py: two async deadline tests (loop-top break; sleep clipped to remaining budget)
• async_client.py: UserWarning for insecure+token; get_server_version() caching; check_version=True lazy path
• async_watcher.py: spurious-wake in changes(); stream loop body; AioRpcError while stopped
• watcher.py: queue-cond wait when empty; stream loop body; RpcError while stop_event set
• make lint passes (ruff check + format)
• make test: 282 passed, 13 skipped, total coverage 100%

Closes #112

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!