fix: Don't cache timeout/error results in LeanProver

[haasonsaas]

Summary

• Move cache.put inside the try block so only actual lean execution results (success/type-check failure) get cached
• Timeout and unexpected errors are transient — caching them permanently prevents retries
• Matches CoqProver behavior

Addresses Bugbot review on #3.

Test plan

• 288 tests pass
• CI lint clean

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Low risk control-flow change limited to LeanProver.prove_specification, preventing transient timeout/unexpected-error results from being written to ProofCache while leaving successful/typecheck-failure caching unchanged.

Overview
Adjusts LeanProver.prove_specification to return immediately on subprocess.TimeoutExpired and other exceptions instead of assigning to result and falling through.

This ensures only actual Lean execution outcomes are cached via ProofCache.put, and transient timeouts/unknown failures no longer poison the cache and block retries.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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