feat: warn when brotli extra missing

[tomasgesino]

Summary

feat: warn when brotli extra missing

• add explicit warning if server sends Content-Encoding: br without brotli support
• surface optional feature flags in decoders and ensure behavior covered by test
• document this repo's contribution workflow for agents in AGENTS.md

PR description:

Summary

• raise a clear warning when responses arrive with Content-Encoding: br but brotli support isn't installed to avoid silent compressed payloads
• expose decoder availability via flags to share state with response handling
• document contributor workflow and add a regression test covering the warning path

Testing

• scripts/install
• scripts/test

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.