feat(ws): implement bidirectional WebSocket proxy (#2390)

* feat(ws): implement bidirectional WebSocket proxy

- Add WebSocketProxy struct with builder pattern
- Support custom headers for upstream auth via ClientRequestBuilder
- Implement bidirectional message forwarding between client and upstream
- Add pending message queue with control vs data message separation
- Implement backpressure limit (5 MiB MAX_PENDING_QUEUE_BYTES)
- Add upstream connection timeout (5s default)
- Implement graceful shutdown with close code normalization
- Add axum dependency with ws feature for server feature

Co-Authored-By: yujonglee <[email protected]>

* feat(ws): add tower::Service, preconnect API, and friendlier builder

- Implement tower::Service for plug-and-play Axum routing
- Add preconnect() method for latency-sensitive scenarios
- Add friendlier builder API: .upstream_url(), .header(), .headers()
- Add PreconnectedProxy for handling pre-established upstream connections
- Add PreconnectError enum for preconnect error handling
- Add handle_upgrade() method for WebSocketUpgrade handling
- Add tower dependency for server feature

Co-Authored-By: yujonglee <[email protected]>

* chore: update Cargo.lock for tower dependency

Co-Authored-By: yujonglee <[email protected]>

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: yujonglee <[email protected]>

Author: devin-ai-integration[bot]

Cargo.lock

@@ -6,7 +6,7 @@ edition = "2024"
 [features]
 default = ["client"]
 client = []
-server = []
+server = ["axum", "tower"]
 
 [dependencies]
 bytes = { workspace = true }
@@ -20,5 +20,8 @@ tokio = { workspace = true, features = ["rt-multi-thread", "time", "sync", "macr
 tokio-tungstenite = { workspace = true, features = ["native-tls-vendored"] }
 tracing = { workspace = true }
 
+axum = { workspace = true, optional = true, features = ["ws"] }
+tower = { workspace = true, optional = true }
+
 [dev-dependencies]
 serde_json.workspace = true