test: simple, lightweight, dependency-free Python proxy server

[jpnurmi]

Replace mitmproxy with a simple, lightweight, dependency-free Python proxy server for integration tests. Besides fixing the unstable proxy tests, this allows radically simplifying the custom Alpine image by eliminating the manual build of mitmdump and its dependencies, which currently takes about two hours. Supports HTTP proxy, proxy-auth, IPv6 bind, and SOCKS5 cases covered by the test suite.

See also:

• test: stabilize proxy tests #1762
  • https://github.com/getsentry/sentry-native/actions/runs/30931657868/job/92067689146
  • https://github.com/getsentry/sentry-native/actions/runs/30994059904/job/92266775721
  • https://github.com/getsentry/sentry-native/actions/runs/31094910368/job/92594565633
  • https://github.com/getsentry/sentry-native/actions/runs/31589570461/job/94091155479
  • ...
• build(docker): Update Alpine images (3.23 + multi-arch) #1713
  • build(deps): bump mitmproxy from 11.0.0 to 12.2.2 in /tests #1643
  • build(deps): bump msgpack from 1.0.8 to 1.2.1 in /tests #1821
  • ci: fix, update, and clean up alpine docker image #1941
  • https://github.com/getsentry/sentry-native/actions/runs/30893425596 (~2h!)

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.27%. Comparing base (0626940) to head (20609f8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1967      +/-   ##
==========================================
- Coverage   74.32%   74.27%   -0.05%     
==========================================
  Files         104      104              
  Lines       25701    25701              
  Branches     4640     4640              
==========================================
- Hits        19101    19089      -12     
- Misses       5293     5305      +12     
  Partials     1307     1307              

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