fix: honor zero content length

[BunsDev]

Summary

• Treat parsed CONTENT_LENGTH=0 as an empty request body.
• Keep missing or unparsable content lengths on the bounded EOF-read path.
• Add regression coverage for the zero-length WSGI case raised by Copilot on fix: address webhook review followups #3.

Verification

• python3 -m unittest discover -s tests -v
• python3 -m py_compile coven_github_adapter.py tests/test_webhook_adapter.py
• git diff --check
• scripts/smoke-webhook.sh http://127.0.0.1:8765/webhook smoke-secret

[Copilot]

Pull request overview

This PR adjusts the webhook WSGI request-body reader to correctly treat a parsed CONTENT_LENGTH=0 as an empty body (instead of falling back to an EOF read), and adds a regression test to cover that case. This keeps the adapter a thin, predictable forwarder while hardening a real-world edge case in WSGI environments.

Changes:

• Treat CONTENT_LENGTH=0 as a valid length and read exactly 0 bytes from wsgi.input.
• Preserve existing behavior for missing/unparsable CONTENT_LENGTH by keeping the bounded EOF-read path.
• Add a regression unit test covering the zero-length content scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
coven_github_adapter.py	Adjusts read_request_body to honor CONTENT_LENGTH=0 by using a fixed-length read.
tests/test_webhook_adapter.py	Adds a unit test validating the zero-content-length behavior and expected 400 invalid json response.

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