fix(codecompanion): improve error handling for empty stderr output

[ruslanfialkovskii]

• Enhanced flatten_table_to_string to filter out empty error messages
• Added logic to distinguish actual errors from empty stderr output
• Prevents false error reporting when VectorCode operations succeed but have no stderr
• Provides meaningful fallback error message for truly empty error conditions
• Fixes issue where successful operations with empty stderr were reported as { "" } errors

Resolves the confusing empty error message issue in CodeCompanion integration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (af52fed) to head (c131596).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #286   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          25       25           
  Lines        1838     1838           
=======================================
  Hits         1833     1833           
  Misses          5        5           

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[Davidyz]

Thanks for the contribution. Please review my comments (mostly about reusing code where possible) and lmk what you think about them. Happy to merge once we're in agreement about those.