fix(rustproxy-http): improve HTTP/3 connection reuse and clean up stale proxy state

rust/crates/rustproxy-http/src/connection_pool.rs

@@ -56,7 +56,11 @@ struct PooledH2 {
 }
 
 /// A pooled QUIC/HTTP/3 connection (multiplexed like H2).
+/// Stores the h3 `SendRequest` handle so pool hits skip the h3 SETTINGS handshake.
 pub struct PooledH3 {
+    /// Multiplexed h3 request handle — clone to open a new stream.
+    pub send_request: h3::client::SendRequest<h3_quinn::OpenStreams, Bytes>,
+    /// Raw QUIC connection — kept for liveness probing (close_reason) only.
     pub connection: quinn::Connection,
     pub created_at: Instant,
     pub generation: u64,
@@ -197,7 +201,10 @@ impl ConnectionPool {
 
     /// Try to get a pooled QUIC connection for the given key.
     /// QUIC connections are multiplexed — the connection is shared, not removed.
-    pub fn checkout_h3(&self, key: &PoolKey) -> Option<(quinn::Connection, Duration)> {
+    pub fn checkout_h3(
+        &self,
+        key: &PoolKey,
+    ) -> Option<(h3::client::SendRequest<h3_quinn::OpenStreams, Bytes>, quinn::Connection, Duration)> {
         let entry = self.h3_pool.get(key)?;
         let pooled = entry.value();
         let age = pooled.created_at.elapsed();
@@ -215,13 +222,20 @@ impl ConnectionPool {
             return None;
         }
 
-        Some((pooled.connection.clone(), age))
+        Some((pooled.send_request.clone(), pooled.connection.clone(), age))
     }
 
-    /// Register a QUIC connection in the pool. Returns the generation ID.
-    pub fn register_h3(&self, key: PoolKey, connection: quinn::Connection) -> u64 {
+    /// Register a QUIC connection and its h3 SendRequest handle in the pool.
+    /// Returns the generation ID.
+    pub fn register_h3(
+        &self,
+        key: PoolKey,
+        connection: quinn::Connection,
+        send_request: h3::client::SendRequest<h3_quinn::OpenStreams, Bytes>,
+    ) -> u64 {
         let gen = self.h2_generation.fetch_add(1, Ordering::Relaxed);
         self.h3_pool.insert(key, PooledH3 {
+            send_request,
             connection,
             created_at: Instant::now(),
             generation: gen,