Raise upstream request timeout to 300s.

Forwarded chat/responses/messages requests used URLSession's 60s default, so
large-context and reasoning requests whose first-byte latency exceeds 60s failed
with a spurious 502 timeout. Give the upstream request a 300s timeout.

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

Author: cailurus

Sources/CopilotBridge/Proxy/CopilotUpstream.swift

@@ -176,6 +176,9 @@ actor CopilotUpstream {
         let token = try await tokens.get()
         var req = URLRequest(url: endpoint(path, base: token.apiBaseURL))
         req.httpMethod = "POST"
+        // Large contexts and reasoning models can take well over the URLSession default
+        // (60s) to produce the first byte; a short timeout surfaces as a spurious 502.
+        req.timeoutInterval = 300
         for (k, v) in headers(token.value) { req.setValue(v, forHTTPHeaderField: k) }
         for (k, v) in extraHeaders { req.setValue(v, forHTTPHeaderField: k) }
         req.setValue("application/json", forHTTPHeaderField: "Content-Type")