Keep HTTP/2 when takeover relays Cursor streaming calls

[donalffons]

Summary

• Cursor speech-to-text (and the network diagnostic's Agent check) uses a bidirectional HTTP/2 Connect stream. With takeover enabled, that check waits 60s and dictation records a waveform but never inserts text. With takeover disabled, the same Agent check succeeds.
• Stock hudsucker 0.25 rewrites every proxied request to HTTP/1.1 in normalize_request. HTTP/1.1 cannot carry that stream: the upstream does not send the first response until the request body ends, and the client will not end the body until the first response arrives.
• Vendor hudsucker 0.25.0 and keep HTTP/2 for HTTPS upstreams. Plaintext HTTP (the local backend) still downgrades to HTTP/1.1, so locally handled agent routes are unchanged.

Test plan

• cargo test --manifest-path third_party/hudsucker/Cargo.toml --lib proxy::internal::tests::normalize_request (preserves HTTP/2 for https, downgrades plaintext http)
• Quit the running Cursor BYOK app, start this build, enable takeover, then rerun Cursor's network diagnostic. Agent should pass.
• Dictate a sentence into Cursor's chat input and confirm the transcript appears.
• Send a normal agent message with takeover on and confirm model routing still works.