fix(ui): seed RTK Query cache after create/update mutations

[jgaul3]

Description

When I use flipt, I find I have to refresh a time or two after making changes for those changes to be reflected in the UI. I'm quite certain this is a read-after-write bug arising from using invalidatesTags rather than updating those tags. This PR updates them to generally use onQueryStarted instead.

This applies to creating, updating, and deleting flags and segments, as well as creating branches.

I tested things out locally but since it's partially a network thing that's mostly to confirm that basic processes still work.

[github-actions]

Verdict: approve

This PR cleanly replaces RTK Query tag invalidation with explicit cache seeding via onQueryStarted to fix the read-after-write consistency issues described. The pattern is applied consistently across flags, segments, and branch environments; return types are corrected to match actual API responses; duplication is reduced with enrichFlag/enrichSegment helpers; and Playwright tests are strengthened to verify immediate UI consistency. No issues found.

🤖 Automated review by the Flipt PR review agent.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.27%. Comparing base (555c557) to head (7d200dc).
⚠️ Report is 3 commits behind head on v2.

Additional details and impacted files

@@           Coverage Diff           @@
##               v2    #6217   +/-   ##
=======================================
  Coverage   62.27%   62.27%           
=======================================
  Files         145      145           
  Lines       14815    14815           
=======================================
  Hits         9226     9226           
  Misses       4829     4829           
  Partials      760      760           

Flag	Coverage Δ
integrationtests	33.50% <ø> (ø)
unittests	53.85% <ø> (ø)

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