Reject transport-changing extensions

[mjc]

Summary

• classify transport-changing extensions as locally rejected commands
• return a clear 503 response without forwarding them to any backend

Branch-added tests

• A routing-mode matrix sends COMPRESS through hybrid, per-command, and stateful paths and verifies each receives 503 with no backend forwarding.
• Existing command classification and local-response coverage remains green.

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[Copilot]

🟡 Changes recommended

STARTTLS remains a transport-changing command but still falls back to a generic reject response path, so transport-changing commands do not consistently return the clearer transport-specific 503 message.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR extends the proxy’s NNTP command classification to recognize the COMPRESS verb as a transport-changing command and ensures it is locally rejected with a 503 response rather than being forwarded to any backend.

Changes:

• Added RequestKind::Compress and classified COMPRESS as RequestRouteClass::Reject.
• Introduced a dedicated local reject response (TRANSPORT_REJECT) and applied it to COMPRESS in the command handler.
• Added unit tests covering COMPRESS classification/routing and rejection behavior across routing modes.

File summaries

File	Description
src/protocol/request.rs	Adds COMPRESS request kind classification and marks it as locally rejected.
src/command/handler.rs	Adds a transport-specific 503 reject response and tests that COMPRESS is rejected across routing modes.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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