Make source search failures safe

[5aumit]

Outcome

A failed or unavailable optional source does not end the whole SetScout Run. The Run continues with results from healthy sources and reports a clear coverage warning.

Scope

• Skip Kaggle cleanly when credentials are not configured.
• Contain SystemExit and ordinary source exceptions inside the source-search boundary.
• Preserve results from other configured sources.
• Keep raw exception details out of user-facing Run state.
• Add regression tests for missing Kaggle credentials and a source task that raises SystemExit.

Out of scope

• Stop or cancellation controls.
• End-to-end time limits.
• New dataset sources.

Blocked by

Nothing.

[5aumit]

Current-code references

• Kaggle search is started as an independent task: setscout/tools/search.py:120-126.
• The current wrapper catches Exception only: setscout/tools/search.py:128-138. SystemExit is not an Exception, so this boundary does not contain it.

This is the current behavior this ticket must make safe.