[tune] Fix resume="AUTO" compatibility with the new ResumeConfig implementation

[justinvyu]

Why are these changes needed?

#42853 updated the internal implementation to be centered around ResumeConfig, and the other resume options are now translated to a valid ResumeConfig. For example, resume="AUTO+ERRORED" --> ResumeConfig(errored=RESUME).

The bug: resume="AUTO" should map to ResumeConfig(), but it was not being handled properly by the adapter code. This PR fixes that issue and also adds a test so that CI will catch issues in the future.

resume=True behavior change

This bug also highlights a slight behavior change of resume=True. Prior to the PR mentioned above, this configuration would raise an error if no previous experiment was found at {storage_path}/{name}. Now, tune.run(resume=True) behaves the same as tune.run(resume="AUTO") and will start a new run if a previous one is not found.

Tuner.restore(...) is not impacted in the same way, because that API is explicit and will raise an error immediately if the experiment checkpoint is not found.

Related issue number

Closes #43160

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