Remove getattr calls in async transport closing

[adamtheturtle]

Summary

• Add aclose to the AsyncTransport protocol so it's a required method
• Replace getattr-based conditional closing with direct await self._transport.aclose() in all async clients
• Remove tests for transports without aclose, since the protocol now requires it

Test plan

• Pre-commit hooks pass (mypy, pyright, ruff, etc.)

🤖 Generated with Claude Code

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Note

Medium Risk
This is a small but potentially breaking API change for consumers providing custom async transports, which must now implement aclose() or client cleanup will fail.

Overview
Async client cleanup is simplified by removing conditional getattr checks and always calling await self._transport.aclose() in AsyncVWS, AsyncCloudRecoService, and AsyncVuMarkService.

AsyncTransport is tightened to require an aclose() method, and the test coverage for transports without aclose is removed accordingly.

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