[core][autoscaler][v2] Fix invalid default stats factory in ClusterStatus

[weimingdiit]

Description

ClusterStatus.stats currently uses field(default_factory=Stats), but
Stats requires the positional argument gcs_request_time_s.

As a result, ClusterStatus() cannot be default-constructed and raises:

TypeError: Stats.__init__() missing 1 required positional argument: 'gcs_request_time_s'

This PR fixes the invalid default by making ClusterStatus construct a valid default Stats object, and adds a regression test to cover the empty/default construction path.

This is a small schema-level fix and does not change the normal populated paths where callers already pass an explicit Stats(...) instance.

Related issues

#62933

Additional information

Implementation notes:
update ClusterStatus.stats to use a valid default Stats value add a regression test for ClusterStatus()

[gemini-code-assist]

Code Review

This pull request updates the ClusterStatus schema to initialize the stats field with a default gcs_request_time_s value of 0.0 and adds a unit test to verify these defaults. Feedback was provided to improve PEP 8 compliance by ensuring top-level functions in the test file are separated by two blank lines.

[github-actions]

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[rueian]

Hi @edoakes, the previous auto-merge was disabled. Please merge this when you get a chance. Thanks!