Lower ASGI gzip compresslevel from 9 to 4

[anth-volk]

Fixes #3734

Summary

Stage 2 of the Cloud Run cutover failed its warm-parity gate at 3.25× (limit 1.20×), localized entirely to /us/metadata: p95 13.2 s on Cloud Run vs 4.3 s on App Engine, while liveness/readiness are faster on Cloud Run. The ~70 MB raw metadata payload is gzipped in-request by starlette's GZipMiddleware at its default compresslevel=9 on Cloud Run; App Engine flex never pays this cost because its nginx sidecar handles gzip natively at a low level.

Change

One line in asgi_factory.py: explicit compresslevel=4.

Measured on the actual production metadata payload (70.8 MB):

Level	Compress time	Output
9 (current default)	5.46 s	9.0 MB
6	1.33 s	9.1 MB
4 (this PR)	0.73 s	9.9 MB
1	0.40 s	10.9 MB

7.5× faster compression for ~10% larger output (~0.2 s extra transfer at typical bandwidth vs ~5 s of CPU saved per request).

Verification

• tests/unit/test_asgi_factory.py: 15/15 pass; ruff clean.
• Empirical projection: CR metadata TTFB ≈ 9.8 s − (level-9 − level-4 compression delta on Cloud Run CPU) → expected to land near GAE's ~2–4 s.
• Definitive check after merge+deploy: re-run the Stage 2 Phase A parity capture (capture_migration_baseline.py --repetitions 10 both platforms) — gate is CR p95 ≤ GAE p95 × 1.20.

Out of scope per maintainer: the 70 MB payload itself (metadata slimming planned separately) and the Accept-Encoding: identity 500 (Cloud Run ~32 MB response cap; single known consumer always sends gzip; mooted by slimming).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.82%. Comparing base (74c146f) to head (a7f5e46).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3735   +/-   ##
=======================================
  Coverage   79.82%   79.82%           
=======================================
  Files          70       70           
  Lines        4336     4336           
  Branches      808      808           
=======================================
  Hits         3461     3461           
  Misses        655      655           
  Partials      220      220           

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