Fix race condition in timeout handler causing activator crashes

[Fedosin]

Fixes #15850

Proposed Changes

Add synchronization for HTTP header map access during request timeouts. When a timeout occurs, capture a snapshot of headers to prevent concurrent modification by the inner handler after the timeout handler returns.

Release Note

NONE

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 80.10%. Comparing base (41fafd1) to head (a2e2dde). :warning: Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/http/handler/timeout.go	76.92%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16222      +/-   ##
==========================================
+ Coverage   80.04%   80.10%   +0.06%     
==========================================
  Files         214      214              
  Lines       13313    13325      +12     
==========================================
+ Hits        10656    10674      +18     
+ Misses       2296     2292       -4     
+ Partials      361      359       -2     

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