fix(types): Validate groupId in installation external issue actions

[JoshFerge]

Summary

Add validation to ensure groupId is provided in request data, returning 400 error if missing before querying the Group model.

Changes

• Add None check for group_id after extracting from request data
• Return 400 error with proper field validation message
• Type-safe query to Group model

Test plan

• Existing tests pass
• Mypy type checking passes

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...i/endpoints/installation_external_issue_actions.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #103026      +/-   ##
===========================================
- Coverage   80.75%    80.64%   -0.11%     
===========================================
  Files        9145      9140       -5     
  Lines      395020    392795    -2225     
  Branches    24968     24968              
===========================================
- Hits       319002    316775    -2227     
- Misses      75617     75619       +2     
  Partials      401       401