fix(pubsub): handle internal metadata changes in google-api-core

[chalmerlowe]

Problem

The test_init_default_client_info tests in google-cloud-pubsub (for both Publisher and Subscriber) were brittle because they peeked into the private internal attributes of _GapicCallable (from google-api-core) to verify metadata.
Recent refactoring in google-api-core renamed/removed the _metadata attribute, causing these tests to fail with AttributeError: '_GapicCallable' object has no attribute '_metadata'.

Solution

Refactored test_init_default_client_info in test_publisher_client.py and test_subscriber_client.py to use mocking instead of peeking into internal state.

• The tests now use unittest.mock.patch to intercept calls to google.api_core.gapic_v1.method.wrap_method.
• We verify that wrap_method is called with the expected client_info object.
• We assert that the client_info contains the correct library version string (e.g., gccl/x.y.z).

Why this is important

• Robustness: Tests are no longer dependent on internal implementation details or private attributes of downstream libraries (google-api-core).
• Future-Proof: Prevents test failures when internal state is refactored, as long as the public-facing helper signature (wrap_method) remains stable.
• Consistency: Aligns with the testing patterns already adopted by other modern clients in this monorepo (e.g., google-cloud-bigquery-storage).

Blocks: #17642

[gemini-code-assist]

Code Review

This pull request updates the unit tests for both the publisher and subscriber clients to handle internal changes in the google-api-core library's _GapicCallable. Specifically, it safely retrieves metadata from wrapped methods by falling back to _default_metadata if _metadata is not present. Additionally, some minor import cleanups were performed. There are no review comments, and I have no feedback to provide.