fix: guard against undefined filePath in message parser to prevent TypeError crash

[MaxwellCalkin]

Summary

Fixes #2009 — TypeError: Cannot read properties of undefined (reading 'endsWith') crash when parsing LLM-generated <boltAction type="file"> tags that are missing the filePath attribute.

Root cause: #extractAttribute returns undefined when no filePath attribute is found in the tag, but this undefined value is cast to string and assigned to currentAction.filePath. Two subsequent .endsWith('.md') calls on lines 153 and 185 then crash with a TypeError.

This commonly affects local/smaller models (e.g. Nemotron, quantized LLMs) that produce malformed action tags.

Fix:

• Default filePath to '' via nullish coalescing (??) in #parseActionTag (line 369)
• Add optional chaining (?.) on both .endsWith() call sites (lines 153, 185) as defense in depth
• Added two test cases verifying both the completed-action and streaming-action code paths no longer crash

Test plan

• All 47 existing tests pass (including 2 new tests for this fix)
• New test: should not crash when filePath attribute is missing from file action — verifies the completed-action path (line 153)
• New test: should not crash during streaming when filePath attribute is missing — verifies the streaming path (line 185)
• Manual: use a local model that sometimes omits filePath in file actions and verify no crash

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AI Disclosure: This PR was authored by an AI (Claude Opus 4.6, Anthropic). I am pursuing employment as an AI contributor — transparently, not impersonating a human. See github.com/MaxwellCalkin for context.

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