fix: render ticket timeline legend from what the rail draws

[RitvikSardana]

Timeline legend was static, so it listed colors the rail never drew. Now it renders only what is on the rail, adds the black Pending dot, and names red for its actual cause (SLA missed, Longest wait, or both).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.76%. Comparing base (c3ae467) to head (fe2ebae).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3697   +/-   ##
========================================
  Coverage    73.76%   73.76%           
========================================
  Files          148      148           
  Lines        10841    10841           
========================================
  Hits          7997     7997           
  Misses        2844     2844           

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The computed legend remains reactive and consistently represents every labeled color rendered by the timeline rail.

_{Reviews (1): Last reviewed commit: "fix: render timeline legend from what th..." | Re-trigger Greptile}