Fix ZlibError by removing compression headers after httpx decompression

[pythoninthegrass]

Description

httpx automatically decompresses responses when reading response.content, but the proxy was forwarding the original Content-Encoding headers. This caused clients to attempt double-decompression, resulting in ZlibError.

Fixes #2

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

Fixes compression header handling in:

• Non-streaming Anthropic/OpenAI responses
• Cached responses
• Passthrough handler

Testing

Describe the tests you ran to verify your changes:

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

Test Output

λ uv run pytest tests/test_proxy_compression_headers.py -v
==================================================================== test session starts =====================================================================
platform darwin -- Python 3.13.5, pytest-9.0.2, pluggy-1.6.0 -- /Users/lance/git/headroom/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /Users/lance/git/headroom
configfile: pyproject.toml
plugins: anyio-4.12.1, asyncio-1.3.0, cov-7.0.0
asyncio: mode=Mode.AUTO, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 6 items

tests/test_proxy_compression_headers.py::TestCompressionHeaderRemoval::test_compression_headers_are_removed_from_dict PASSED                           [ 16%]
tests/test_proxy_compression_headers.py::TestCompressionHeaderRemoval::test_response_body_is_decompressed_not_compressed PASSED                        [ 33%]
tests/test_proxy_compression_headers.py::TestCompressionHeaderRemoval::test_headers_with_wrong_content_length_cause_issues PASSED                      [ 50%]
tests/test_proxy_compression_headers.py::TestCompressionHeaderRemoval::test_removing_headers_fixes_length_mismatch PASSED                              [ 66%]
tests/test_proxy_compression_headers.py::TestNoRegressionForUncompressedResponses::test_pop_on_missing_keys_is_safe PASSED                             [ 83%]
tests/test_proxy_compression_headers.py::TestNoRegressionForUncompressedResponses::test_dict_conversion_preserves_headers PASSED                       [100%]

===================================================================== 6 passed in 1.54s ======================================================================

Checklist

To qualify the below, inline comments and existing errors weren't relevant to the fix.

• 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

Screenshots (if applicable)

N/A

Additional Notes

You should consider release-please by Google re: changelogs.