fix: Catch all exceptions from _run_lean to prevent UnboundLocalError

[haasonsaas]

Summary

• Catches all exceptions from _run_lean, not just TimeoutExpired, preventing UnboundLocalError when e.g. UnicodeDecodeError occurs from non-UTF-8 lean output
• Returns INCONCLUSIVE result instead of crashing

Addresses Sentry review feedback on #3.

Test plan

• 288 tests pass
• CI lint clean
• CI green

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Low risk: only broadens error handling around Lean invocation to prevent crashes; behavior change is limited to returning an INCONCLUSIVE ProofResult (and logging) for unexpected exceptions.

Overview
Prevents LeanProver.prove_specification from crashing when _run_lean raises non-timeout exceptions (e.g., decode/subprocess issues) by catching all exceptions, logging the failure, and returning a non-proven ProofResult with solver_status=INCONCLUSIVE.

Timeout handling and caching behavior are unchanged; unexpected failures now produce a structured result with elapsed time and the exception message instead of propagating.

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