## Why
Configured HTTP credential destinations need dummy credentials translated inside `CONNECT` and SOCKS5 tunnels, where credential interception previously detected only TLS.
## What changed
- Detect plaintext HTTP for configured credential destinations and proxy requests with URL-scoped credential substitution.
- Keep requests bound to the authorized tunnel destination, rejecting mismatched authorities and nested `CONNECT` requests.
- Support HTTP/2 clients and preserve HTTP upgrades, including `h2c`, and opaque traffic in full mode.
- Enforce limited-mode method restrictions on plaintext HTTP and reject upgrades and opaque traffic.
## Testing
Add end-to-end tests for both tunnel transports covering credential URL scope, request bodies, HTTP/2, destination mismatches, upgrades, lossless opaque forwarding, and limited-mode enforcement.
GitOrigin-RevId: 89262c996ea4ee156c628f9ed8e72b52504b2f6b