ci: add kit contract tests action

[doc-han]

Description

Adds a CI action for checking contract tests between lightning(current branch) and kit.

Validation steps

1. (How can a reviewer validate your work?)

Additional notes for the reviewer

1. (Is there anything else the reviewer should know or look out for?)

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just
want to know!):

• I have used Claude Code
• I have used another model
• I have not used AI

You can read more details in our
Responsible AI Policy

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review
  with Claude Code)
• I have implemented and tested all related authorization policies.
  (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): N/A, PR only adds a GitHub Actions workflow file (.github/workflows/kit-contract.yml:1) with no application data access.
• S1 (authorization): N/A, no web-layer actions, controllers, LiveView events, or policy modules are added or modified.
• S2 (audit trail): N/A, no writes to project or instance configuration resources are introduced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.9%. Comparing base (ffa1d38) to head (c8beeb9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5190     +/-   ##
=======================================
- Coverage   90.9%   90.9%   -0.1%     
=======================================
  Files        425     425             
  Lines      21156   21156             
=======================================
- Hits       19241   19230     -11     
- Misses      1915    1926     +11     

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