Fix progress bar estimation for FT Jobs

[artek0chumak]

This PR fixes time estimations for the Finetuning jobs. Instead of using update_at timestamp, it uses started_at timestamp. The new timestamp indicates when the current stage of the training job has started.

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Note

Switches progress/ETA calculations to the job started_at timestamp for accuracy.

• Update generate_progress_bar to gate on and derive elapsed time from started_at (with timezone-safe parsing) instead of updated_at
• Extend FinetuneResponse with started_at and wire it into progress logic
• Revise unit tests to construct responses with started_at, adjust assertions (including timezone, future-start, and rich formatting cases)

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