## Why
Policy evaluation and upstream parsers must not select different `Host` field lines from the same request.
## What changed
Return `400 Bad Request` for multiple `Host` headers, including identical values, before policy evaluation, upstream forwarding, or Unix socket routing. Apply the check to both plain HTTP requests and `CONNECT` requests.
## Testing
Add regression tests covering identical and conflicting headers in origin-form, absolute-form, and `CONNECT` requests, verifying rejection without an upstream connection. Also cover single-header forwarding and rejection before Unix socket routing.
GitOrigin-RevId: 8ec95171e120f784629d3121b9a1bedbaf88e7cd
