refactor: move shared job utilities to common module

[Debashich]

What this PR does / why we need it

Implements KEP-107 by moving shared job-submission utilities used by the Trainer, Optimizer, and Spark clients into kubeflow.common.

Changes

• Adds shared Kubernetes utilities for:

  • Job event retrieval
  • Pod log retrieval
  • Random name generation

• Adds a shared Event dataclass to kubeflow.common.types.

• Refactors Trainer, Optimizer, and Spark backends to use the common utilities.

• Re-exports Event through Trainer types for compatibility.

• Removes duplicated utility implementations and redundant tests.

• Adds unit tests covering shared utilities, timeout handling, and pod log streaming.

This reduces duplication across the three clients and improves long-term maintainability and consistency.

Testing

• make test-python — 642 tests passed
• make verify — All checks passed

Which issue(s) this PR fixes

Fixes #608

Checklist

• [Docs](https://www.kubeflow.org/docs/) included if any changes are user facing

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[github-actions]

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[Debashich]

/cc @tariq-hasan @andreyvelich

The PR is ready for review. I’ve implemented KEP-107, added tests for the new shared common utilities, and verified the full test suite (642 passed) along with make verify.

Please take a look when you’re free. Thanks!