feat(trainer): improve DX for LocalProcessBackend missing runtime error

[zannensk]

What this PR does / why we need it:
While exploring the LocalProcessBackend as part of my GSoC proposal preparation, I noticed that the error message when a runtime is missing is quite opaque (ValueError: Runtime must be provided).

This PR improves the Developer Experience (DX) by dynamically listing the available runtimes (e.g., 'torch-distributed') within the error message. This helps newcomers quickly identify the correct runtime string without deep-diving into the source code.

Checklist:

• [Docs] included if any changes are user facing (Updated the error message itself)
• I have run make verify and it passed.
• I have run make test-python and all tests passed.

[google-oss-prow]

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[Copilot]

Pull request overview

Improves the LocalProcessBackend developer experience by enhancing the “missing runtime” error to include a dynamically generated list of available runtime names, and updates the corresponding unit test expectation.

Changes:

• Extend the LocalProcessBackend.train() ValueError message to list available runtimes when runtime is None.
• Update backend_test.py to assert the new, more informative error message prefix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
kubeflow/trainer/backends/localprocess/backend.py	Adds available runtime names to the “runtime missing” exception message in train().
kubeflow/trainer/backends/localprocess/backend_test.py	Updates the failing-case assertion to match the new error message prefix.