Make test synchronization deterministic

[quinnj]

Summary

• replace test-owned sleeps, polling deadlines, and elapsed-time assertions with explicit lifecycle signals
• use exact protocol reads and injected clock and wait functions for timeout behavior
• add contributor guidance and an enforcement test for deterministic synchronization
• remove the timing watchdog and Windows timing warmup while retaining the workflow job timeout as the final deadlock guard

Root cause

GitHub Actions can pause Julia tasks for unpredictable periods. The test suite used short sleeps, polling deadlines, and elapsed-time limits as proxies for protocol state. Runner scheduling delays could therefore fail correct HTTP behavior. Some timing guards could also hide a real deadlock instead of identifying the missing lifecycle signal.

The first CI run also exposed a coverage regression. Some rewritten deadline tests used already-expired deadlines and no longer executed active wait and wake paths. The follow-up adds deterministic lifecycle barriers and private clock and wait seams. It restores these paths without wall-clock coordination.

Validation

• full Julia 1.12.6 package test with coverage, including all 63 trim checks
• Julia 1.10.11 policy, HTTP core, HTTP/1 transport and server, HTTP/2 client and server, and WebSocket suites
• local source coverage increased from 92.3576% at the base commit to 92.3865% at this head
• git diff --check
• exact source scan for prohibited test-owned clock waits

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (4b6c835) to head (eca220c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1346   +/-   ##
=======================================
  Coverage   88.51%   88.52%           
=======================================
  Files          31       31           
  Lines       11955    11961    +6     
=======================================
+ Hits        10582    10588    +6     
  Misses       1373     1373           

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