fix: replace mutable default set with None in wait_for_job_status

[Roaimkhan]

What this PR does / why we need it:
Python evaluates default argument values once at function definition time. Using a mutable set literal as a default means all callers share the same object — if it's ever mutated, every subsequent call using the default sees the corrupted state.

This PR fixes all instances of this pattern across the trainer and optimizer backends by replacing the mutable default with None and assigning the real value inside the function body, which is the canonical Python solution.
Before:

def wait_for_job_status(
    self,
    name: str,
    status: set[str] = {constants.TRAINJOB_COMPLETE},  # shared mutable object!
    ...

After:

def wait_for_job_status(
    self,
    name: str,
    status: set[str] | None = None,
    ...
) -> types.TrainJob:
    if status is None:
        status = {constants.TRAINJOB_COMPLETE}  # fresh set per call
    ...

Fixes #329

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[Copilot]

Pull request overview

This PR fixes a Python anti-pattern where mutable set literals were used as default arguments in wait_for_job_status methods across trainer and optimizer backends. Using mutable defaults can cause all callers to share the same object, leading to subtle bugs if that object is ever mutated. The fix follows the canonical Python pattern: replace the mutable default with None and assign the real default value inside the function body.

Changes:

• Updated all wait_for_job_status method signatures to use status: set[str] | None = None instead of status: set[str] = {constants.CONSTANT}
• Added if status is None: checks at the beginning of each method to assign the default value
• Updated docstring in trainer_client.py to document the default value

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
kubeflow/trainer/backends/localprocess/backend.py	Fixed mutable default in wait_for_job_status
kubeflow/trainer/backends/kubernetes/backend.py	Fixed mutable default in wait_for_job_status
kubeflow/trainer/backends/container/backend.py	Fixed mutable default in wait_for_job_status
kubeflow/trainer/backends/base.py	Fixed mutable default in abstract base method
kubeflow/trainer/api/trainer_client.py	Fixed mutable default and updated docstring
kubeflow/optimizer/backends/kubernetes/backend.py	Fixed mutable default in wait_for_job_status
kubeflow/optimizer/backends/base.py	Fixed mutable default in abstract base method
kubeflow/optimizer/api/optimizer_client.py	Fixed mutable default in wait_for_job_status