AI Bridge Proxy: handle plain HTTP proxy requests instead of rejecting with 407

[ssncferreira]

Problem

AI Bridge Proxy rejects plain HTTP requests that go through it with a 407 (Proxy Authentication Required). This happens because HTTP requests are sent directly to the proxy without a CONNECT handshake, so they reach handleRequest without a requestContext (which is only set during the CONNECT phase by authMiddleware).

While the proxy should primarily be configured as HTTPS_PROXY (since all supported AI provider domains use HTTPS), users or tools may also set HTTP_PROXY pointing to the same address. When this happens, plain HTTP traffic is routed through the proxy and rejected.

How to reproduce

1. Send a plain HTTP request through the proxy: curl -x http://coder:testtoken@localhost:8888 http://example.com/test
2. Logs show:

[API] 2026-02-12 08:50:21.409 [warn]  coderd.aibridgeproxyd: rejecting request with missing context  host=example.com  method=GET  path=/test
[API] 2026-02-12 08:50:21.409 [warn]  coderd.aibridgeproxyd: received error response from aibridged  connect_id=00000000-0000-0000-0000-000000000000  request_id=00000000-0000-0000-0000-000000000000  provider=""  status=407

Proposal

Update handleRequest so that when reqCtx is nil and the request is plain HTTP (not a decrypted MITM'd HTTPS request), forward it directly to the destination, the same way non-allowlisted CONNECT requests are tunneled via tunneledMiddleware.

Note: Authentication for tunneled traffic (both HTTP and non-allowlisted HTTPS) is tracked in issue: #1352.

[ssncferreira]

This was discovered in coder/registry#725, where setting HTTP_PROXY causes agentapi to route its http requests to Coder through the AI Bridge Proxy. These requests are rejected because they arrive without a CONNECT context:

[API] 2026-02-11 18:20:56.179 [warn]  coderd.aibridgeproxyd: rejecting request with missing context  host=host.docker.internal:3000  method=PATCH  path=/api/v2/workspaceagents/me/app-status
[API] 2026-02-11 18:20:56.179 [warn]  coderd.aibridgeproxyd: received error response from aibridged  connect_id=00000000-0000-0000-0000-000000000000  request_id=00000000-0000-0000-0000-000000000000  provider=""  status=407