chore: unify Python function validation between TrainerClient and SparkClient

[mehulkumaryadav]

Summary

Closes #637

This PR extracts the common Python function validation logic shared by TrainerClient and SparkClient into a reusable utility to ensure consistent validation behavior across both clients.

Changes

• Added validate_python_function() to kubeflow.common.utils
• Moved shared Python function validation logic from SparkClient into the common utility
• Updated TrainerClient to use the shared validation utility
• Kept Spark-specific validation (function arguments and heredoc delimiter checks) in SparkClient
• Added unit tests for the shared validation utility
• Updated Trainer utility and backend tests to use regular Python functions instead of lambda functions

Testing

• Added unit tests for validate_python_function()
• Updated Trainer utility tests
• Updated Trainer backend tests
• Ran project tests and pre-commit hooks

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[lntutor]

The decorated-function test does not exercise the decorator branch. @sample_decorator replaces sample_decorated_function with the nested wrapper; because that wrapper has no @... syntax, inspect.getsource(sample_decorated_function) yields the wrapper source and ast.FunctionDef.decorator_list is empty. The helper therefore accepts it instead of raising the asserted ValueError. Please use a pass-through decorator (def decorator(func): return func) for this test so the inspected function retains the decorated source, or otherwise test the actual callable/source pair that the validator receives.

[mehulkumaryadav]

Thanks , I have addressed this feedback by updating the test to use a pass-through decorator so it exercises the actual decorator branch. The corrected version has been pushed.

[tariq-hasan]

/ok-to-test
/retest

[tariq-hasan]

Hi @mehulkumaryadav! I have added a few comments.

[tariq-hasan]

I think we want to apply this check to localprocess and container backend as well.

[tariq-hasan]

Let's match the Args, Raises, Returns docstring pattern from validate_wait_for_job_status.

[tariq-hasan]

Given the validation we can remove the duplicate logic in utils.py.

[Goku2099]

it might be nice to keep using the same section-divider style (like # Test Helpers / # Tests above) across other test files too, just for consistency when browsing the suite. No strong opinion though.
https://github.com/mehulkumaryadav/sdk/blob/2895b0780e39ece6e1ba9cc94af42f5ed2a99dbb/kubeflow/spark/backends/kubernetes/utils_test.py#L90

[mehulkumaryadav]

@tariq-hasan @Goku2099 I have reviewed and addressed all the review comments. Please let me know if there's anything else you'd like me to update. Thanks!