backend: add single-use token auth for spawned stream proxies

Generate a random 256-bit token when spawning a proxy process, pass
it as a CLI argument, and return it to the client in the stream start
response. The proxy validates the token on WebTransport connect and
consumes it after first use, preventing replay. A wrong token attempt
also consumes the token for security. Includes 5 unit tests for token
validation logic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gamestream-webtransport-proxy/src/proxy/handler.rs

@@ -85,31 +85,17 @@ impl crate::proxy::Proxy {
             description: "Could not start stream".to_string(),
         });
 
-        // Validate single-use stream token
+        // Validate single-use stream token via the shared helper so this
+        // handler and its unit tests exercise the same code path.
         let provided_token = req.query::<String>("token").unwrap_or_default();
-        {
+        if let Err(msg) = super::validate_stream_token(&self, &provided_token).await {
-            let mut token_guard = self.stream_token.write().await;
+            error!("Stream token validation failed: {msg}");
-            match token_guard.take() {
+            return Err(AppError {
-                Some(expected) if expected == provided_token => {
+                status_code: StatusCode::UNAUTHORIZED,
-                    // Token consumed successfully (single-use)
+                description: msg,
-                    info!("Stream token validated and consumed");
+            });
-                }
-                Some(_) => {
-                    error!("Invalid stream token provided");
-                    return Err(AppError {
-                        status_code: StatusCode::UNAUTHORIZED,
-                        description: "Invalid stream token".to_string(),
-                    });
-                }
-                None => {
-                    error!("Stream token already consumed");
-                    return Err(AppError {
-                        status_code: StatusCode::UNAUTHORIZED,
-                        description: "Stream token already used".to_string(),
-                    });
-                }
-            }
         }
+        info!("Stream token validated and consumed");
 
         info!("WebTransport connection initiated");
         let (wt_stream_send, wt_stream_recv, wt_datagram_send) = match setup_webtransport(req).await