fix: replace String().trim() with optional chaining

[avoidwork]

Description

Replace String(existingVariable).trim() with existingVariable?.trim() ?? existingVariable in filesystem and session_search tools. The original pattern performed an unnecessary type coercion before trimming, which could mask null/undefined values.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

• Manual verification of both affected files.
• No functional behavior change — both patterns produce the same result for string inputs.

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied