fix: decode zstd-compressed Codex request bodies

[Anionex]

Summary

Fixes #29.

Codex Desktop compresses larger request bodies with Content-Encoding: zstd. The proxy previously passed those bytes directly to json.loads, so image requests raised UnicodeDecodeError and surfaced as a misleading 502.

This change:

• decodes request bodies before JSON inspection (zstd, gzip / x-gzip, and deflate);
• uses Python 3.14's standard-library zstd decoder when available, otherwise streams through libzstd, with an already-installed zstandard binding as a final compatibility fallback;
• strips Content-Encoding before forwarding the decoded body upstream;
• returns explicit 400 responses for invalid compressed bodies and 415 for unsupported encodings;
• preserves malformed/non-JSON byte passthrough without allowing UnicodeDecodeError to escape;
• adds a regression covering zstd-compressed function_call_output.output image input, including image rewrite and upstream header assertions;
• runs the focused encoding test in Linux and Windows CI jobs.

Verification

Passed locally:

• python3 -m py_compile vision_proxy.py vision_client.py ground.py detect.py bin/glance bin/trace bin/crop skills/vision-tools/scripts/html_shot.py
• python3 tests/test_image_rewrite_shapes.py
• python3 tests/test_focus_hint.py
• python3 tests/test_anthropic_rewrite.py
• python3 tests/test_request_content_encoding.py
• python3 tests/smoke_test_proxy.py
• python3 tests/smoke_test_egress_failover.py
• python3 tests/test_vision_client.py
• python3 tests/test_html_shot.py
• python3 tests/test_restore_ui_playbook.py
• python3 tests/test_trace.py
• python3.11 tests/test_request_content_encoding.py
• python3.14 tests/test_request_content_encoding.py
• python3.14 tests/smoke_test_proxy.py
• git diff --check

The optional vtracer / Pillow CLI execution remained skipped because those optional dependencies are not installed; the trace unit checks passed.

User-path verification

This is a non-UI proxy protocol change. The real affected path was exercised end-to-end with a local HTTP client sending a zstd frame shaped like Codex Desktop's view_image Responses payload. The proxy returned 200, rewrote the input_image, forwarded decoded JSON, and removed Content-Encoding. Invalid zstd returned 400 and unsupported br returned 415.

[chatgpt-codex-connector]

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