Fix pass_through with HTTP proxy

[tboerstad]

This PR addresses an issue where pass_through in a proxy environment cause requests to fail:

$ https_proxy=http://proxy:8080 python
>>> with respx.mock:
...     respx.route().pass_through()
...     httpx.get("https://example.com")
httpx.InvalidURL: Invalid port: '8080example.com:443'

When a proxy is configured, httpcore sends a CONNECT request to establish a tunnel. This request uses authority-form URLs (e.g. example.com:443) with no scheme. When respx intercepts this request and tries to parse it into an httpx.URL, the parser crashes.

This PR simply bypass CONNECT requests at the mocker level, before URL parsing occurs.

Note
This means users cannot mock CONNECT requests (e.g. to simulate a 407 Proxy Authentication Required).

[tboerstad]

First, thank you for making respx, very neat!

This PR is a compromise, a small implementation which I think could solve a real issue for users, namely using this library behind a proxy. I ran into this myself when using Anthropic web based Claude Code offering, which is behind a proxy.

However this also means mocking the proxy CONNECT request will not work properly, which is not great.
I leave it up to the maintainer to make a decision here, full understanding if this PR is closed instead