fix(firestore): preserve async limit_to_last ordering

[Sanjays2402]

Closes #17767

The async limit-to-last path compared a protobuf direction enum with a string, so ascending queries were never reversed before fetching. Compare the enum name, matching the synchronous query path, and cover both directions.

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[gemini-code-assist]

Code Review

This pull request fixes an issue in async_query.py where order.direction was compared directly instead of using its .name attribute. It also updates the corresponding unit test test_asyncquery_get_limit_to_last to be parameterized, verifying the behavior for both ascending and descending directions. There are no review comments, so I have no feedback to provide.