Run HTTP server tasks on the interactive thread pool

[quinnj]

Summary

• schedule HTTP listener, connection, HTTP/2 stream, timeout-handler, server-side SSE, and WebSocket server tasks on Julia's :interactive thread pool
• add a regression test that keeps the server responsive while every default-pool thread runs non-yielding work
• document how to configure and verify an interactive thread, the default-pool fallback, and the need to offload non-yielding handler compute

Why

Reseau's foreign thread performs readiness polling. It does not execute Julia request or handler tasks. HTTP.jl v2 had moved the server task tree to the default pool, so non-yielding default-pool compute could still prevent health checks and other HTTP work from running.

This restores the isolation that HTTP.jl v1 provided. The scheduling choice is intentional and is not configurable. If no interactive thread exists, Julia falls back to the default pool.

Fixes #1342.

Validation

• full test suite on Julia 1.12 with --threads=2,1
• starvation regression with all default-pool threads occupied
• HTTP/1, HTTP/2, timeout-handler, SSE, and WebSocket pool assertions
• Julia 1.10 real-server fallback check with zero interactive threads
• Documenter build, doctests, cross-references, and document checks
• git diff --check

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.44%. Comparing base (8e9b80a) to head (d86edb6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
+ Coverage   88.38%   88.44%   +0.06%     
==========================================
  Files          30       31       +1     
  Lines       11923    11930       +7     
==========================================
+ Hits        10538    10552      +14     
+ Misses       1385     1378       -7     

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