fix: mutable default set argument in wait_for_job_status (#329)

[Adithya-S-01]

Fixes #329

Description
This PR resolves the mutable default argument anti-pattern present in the wait_for_job_status methods across the Trainer and Optimizer APIs. Previously, these methods defined status: set[str] = {constants.TRAINJOB_COMPLETE}, which initializes the set object exactly once when the module is evaluated by the Python interpreter.

Because sets are mutable, this pattern causes the exact same set instance to be shared across all subsequent invocations of wait_for_job_status within the same process. If any subclass or client code were to mutate this argument during execution, it would cause side effects across parallel or future SDK calls, leading to unpredictable job polling behavior.

• Updated the method signatures in 8 files across kubeflow/trainer and kubeflow/optimizer client APIs and backends (LocalProcess, Kubernetes, Container) to use status: set[str] | None = None.
• Shifted the default set instantiation into the function bodies using if status is None: status = {constants.*} to guarantee a fresh, isolated set per invocation.
• Removed unused kubeflow.optimizer.constants.constants imports from optimizer_client.py and optimizer/backends/base.py that were orphaned by removing the constants from the method signatures.
• Verified changes via the existing make test-python suite to ensure test coverage, alongside uv run ruff check and uv run ty check inside make verify for strict type and format compliance.

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[Copilot]

Pull request overview

This PR removes mutable default set arguments from wait_for_job_status APIs across Trainer and Optimizer backends/clients, replacing them with None defaults and initializing to the appropriate “complete” status inside implementations.

Changes:

• Update wait_for_job_status signatures to use status: set[str] | None = None instead of a mutable default set.
• Initialize default status sets inside backend implementations (TRAINJOB_COMPLETE / OPTIMIZATION_JOB_COMPLETE).
• Remove now-unneeded constants imports where they were only used for the previous default argument.

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	Avoid mutable default by defaulting status to None and initializing to TRAINJOB_COMPLETE.
kubeflow/trainer/backends/kubernetes/backend.py	Same change for Kubernetes trainer backend, preserving validation and behavior.
kubeflow/trainer/backends/container/backend.py	Same change for container trainer backend, preserving behavior.
kubeflow/trainer/backends/base.py	Update abstract backend API signature to accept status=None.
kubeflow/trainer/api/trainer_client.py	Update client API signature to accept status=None and forward it.
kubeflow/optimizer/backends/kubernetes/backend.py	Avoid mutable default by initializing status to OPTIMIZATION_JOB_COMPLETE when None.
kubeflow/optimizer/backends/base.py	Update abstract optimizer backend API signature and remove unused constants import.
kubeflow/optimizer/api/optimizer_client.py	Update client API signature and remove unused constants import.