fix: Add null checks in local-memory.ts to prevent role crash

[hggzm]

Summary

Adds defensive null checks to prevent crashes when accessing message properties on empty or boundary arrays.

Changes

• Added len > 0 guard before array access in trim loop
• Added optional chaining (?.) for messages[0].role access
• Added last && guard and bounds check in slice operation

Testing

• Verified with ESLint (passes)
• Prevents crash when memory is empty or during edge-case trimming

Fixes #2500, #2503

[hggzm]

Added comprehensive test suite for LocalMemory class in commit ba3fa9d:

Test Coverage: 81% on local-memory.ts

• 21 tests covering all public methods
• Specific tests for the null safety fixes:
  • Empty array access prevention
  • Optional chaining for messages[0].role
  • Bounds checking in collapse boundary detection

All tests pass. 🎉

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[corinagum]

Thanks for the PR! LGTM. Do you mind rebasing with latest? Otherwise my review will be disregarded.

Fixes microsoft/teams-sdk#2500 and microsoft/teams-sdk#2503