Optimize middleware/proxy per-request hot path without behavior changes

[Copilot]

This proxy sits on the Kubernetes API request path, so avoidable per-request work in middleware and request handling directly impacts latency/throughput. This change reduces repeated auth parsing and hot-path allocations while preserving existing behavior.

• Request-scoped auth resolution cache

  • Added a request-context based resolver (ResolveUserAndGroups) to compute user/groups once per request and reuse across middleware/handlers.
  • Replaced repeated NewHTTP(...).GetUserAndGroups() calls in:
    • internal/webserver/webserver.go
    • internal/webserver/middleware/user_in_group.go

• ******** parsing hot-path improvements**

  • Moved websocket bearer regex compilation to package scope (no per-request MustCompile).
  • Added safe regex match handling before group indexing to avoid panic risk.
  • Switched from ReplaceAll("Bearer ", "") to TrimPrefix("Bearer ").

• Proxy handler allocation/recompute reductions

  • In authorization middleware, reused ResponseRecorder.Result() once and closed body once.
  • In selector handling, computed selector.String() once and reused.
  • In token injection paths, fetched BearerToken() once and reused; removed repeated formatting overhead.

Example (new request-scoped resolver usage):

request, user, groups, err := req.ResolveUserAndGroups(
    request,
    authTypes,
    claim,
    writer,
    ignoredImpersonationGroups,
    impersonationGroupsRegexp,
    skipImpersonationReview,
)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (123ffa2) to head (e814940).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1014   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          1       1           
  Lines        200     200           
=====================================
  Misses       200     200           

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[github-actions]

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