Fix pytest warning about empty match parameter in pytest.raises

[Copilot]

Summary

Fixes pytest warnings that occur when using pytest.raises(..., match="") with an empty string pattern. The warnings were introduced in newer versions of pytest and appear in several test cases.

Problem

The test suite was generating warnings like:

pytest.PytestWarning: matching against an empty string will *always* pass. If you want to check for an empty message you need to pass '^$'. If you don't want to match you should pass `None` or leave out the parameter.

This occurred in the _assert_is_disconnected helper function which was using match=r"" when testing ClientNotConnected exceptions.

Solution

• Removed the match=r"" parameter from pytest.raises(ClientNotConnected) calls since the tests only need to verify the exception type and attributes, not the message content
• Removed duplicate test block that was testing the exact same functionality twice
• Preserved all existing test behavior - the tests still validate exception attributes (server_url and operation) as before

Testing

All previously failing tests now pass without warnings:

• test_base_api_client_init[auto_connect=False]
• test_base_api_client_disconnect
• test_base_api_client_async_context_manager[auto_connect=True]
• test_base_api_client_async_context_manager[auto_connect=False]

The fix is minimal and surgical - it only removes the problematic empty match pattern while maintaining all existing test functionality.

[llucax]

Just for the records this duplicated block looks like a wrong revert. At some point we had 2 blocks but one tested client.stub instead:

• db232fe#diff-0d92063e88430a02df61616c5f16b148b64ac4539d9cb9b8d883d5a23351b110R36

But then we removed the client.stub property as it needs to return a async version of the stub, which can only be done by the sub-class, but it looks like instead of being removed it duplicated the test for client.channel.

• 9744ff1#diff-0d92063e88430a02df61616c5f16b148b64ac4539d9cb9b8d883d5a23351b110R33