feat: add --disable-kompress-fallback to restore legacy PASSTHROUGH fallback

[HMAKT99]

Description

Follow-up to #1046. That PR stopped --disable-kompress from forcing
fallback_strategy = CompressionStrategy.PASSTHROUGH, so ContentRouter's rule-based
passes keep running when the ML model is off. As noted in review, that is a behaviour
change for callers who relied on the old passthrough-everything fallback.

This adds an opt-in --disable-kompress-fallback flag (env
HEADROOM_DISABLE_KOMPRESS_FALLBACK) that, together with --disable-kompress, restores
the previous behaviour by routing fall-through content to PASSTHROUGH. It defaults to
off, so the corrected behaviour from #1046 is unchanged unless a caller explicitly opts
back in. The flag is a no-op unless --disable-kompress is also set.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• headroom/proxy/models.py: added disable_kompress_fallback: bool = False to ProxyConfig.
• headroom/proxy/server.py: when disable_kompress and disable_kompress_fallback are both set, restore router_config.fallback_strategy = CompressionStrategy.PASSTHROUGH (re-adding the CompressionStrategy import); wired the new field through the env factory, the __main__ argparse path (--disable-kompress-fallback), and the /health config payload.
• headroom/cli/proxy.py: added the --disable-kompress-fallback Click option (with HEADROOM_DISABLE_KOMPRESS_FALLBACK envvar) and passed it into ProxyConfig.
• tests/test_proxy_disable_kompress.py: added tests for the flag restoring PASSTHROUGH, for it being a no-op without --disable-kompress, and for the /health config payload exposing the field.
• tests/test_cli_proxy_env.py: added a test that the env factory honours HEADROOM_DISABLE_KOMPRESS_FALLBACK.

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

$ pytest tests/test_proxy_disable_kompress.py -v
collected 5 items

tests/test_proxy_disable_kompress.py::test_disable_kompress_config_keeps_optimization_but_disables_ml_fallback PASSED [ 20%]
tests/test_proxy_disable_kompress.py::test_disable_kompress_defaults_to_existing_kompress_behavior PASSED [ 40%]
tests/test_proxy_disable_kompress.py::test_health_config_reports_disable_kompress_fallback PASSED [ 60%]
tests/test_proxy_disable_kompress.py::test_disable_kompress_fallback_restores_passthrough PASSED [ 80%]
tests/test_proxy_disable_kompress.py::test_disable_kompress_fallback_without_disable_kompress_is_noop PASSED [100%]

5 passed

$ ruff check headroom/proxy/server.py headroom/proxy/models.py headroom/cli/proxy.py tests/
All checks passed!

Real Behavior Proof

• Environment: local clone, Python 3.13.7 venv, headroom core deps + fastapi/uvicorn/httpx[http2].
• Exact command / steps: booted the app in-process with FastAPI TestClient across four flag combinations and inspected both the live ContentRouter config and the /health config payload.
• Observed result: both flags -> enable_kompress=False and fallback_strategy=PASSTHROUGH (/health reports disable_kompress_fallback=true); --disable-kompress alone -> fallback_strategy stays KOMPRESS (the fix: --disable-kompress should not override fallback_strategy to PASSTHROUGH #1046 default, /health reports false); --disable-kompress-fallback alone -> no-op (enable_kompress=True, KOMPRESS); neither flag -> defaults (enable_kompress=True, KOMPRESS).
• Not tested: full live-proxy /stats run against a real LLM backend.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Additional Notes

The flag is intentionally a no-op unless --disable-kompress is also set, mirroring where
the original override lived. Happy to add a short note to the docs/README flag list if you'd
like it documented there.

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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