Refine LLM inference callback handlers

Collapse the HTTP callback seam to SendRequest/sendRequest, replace websocket hooks with per-connection handlers, and update tests to use the forwarding handler model.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/LlmRequestHandler.cs

@@ -22,7 +22,7 @@ namespace GitHub.Copilot.Test.E2E;
 /// <remarks>
 /// <para>
 /// This exercises the public extension surface end to end: a consumer subclasses
-/// <see cref="LlmRequestHandler"/> and overrides <see cref="ForwardAsync"/> to
+/// <see cref="LlmRequestHandler"/> and overrides <see cref="SendRequestAsync"/> to
 /// short-circuit the upstream HTTP call with any <see cref="HttpResponseMessage"/>
 /// it likes. The base class streams that response back to the runtime.
 /// </para>
@@ -46,7 +46,7 @@ internal sealed class RecordingInferenceProvider : LlmRequestHandler
     public IReadOnlyList<InterceptedRequest> InferenceRequests =>
         [.. _records.Where(r => IsInferenceUrl(r.Url))];
 
-    protected override async Task<HttpResponseMessage> ForwardAsync(HttpRequestMessage request, LlmRequestContext ctx)
+    protected override async Task<HttpResponseMessage> SendRequestAsync(HttpRequestMessage request, LlmRequestContext ctx)
     {
         var url = request.RequestUri!.ToString();
         _records.Enqueue(new InterceptedRequest(url, ctx.SessionId));

dotnet/test/E2E/LlmInferenceE2EProvider.cs

@@ -46,23 +46,20 @@ private static LlmInferenceRequest HttpRequest(
         };
 
     /// <summary>A handler whose upstream call is a canned delegate (no network).</summary>
-    private sealed class StubHandler(Func<HttpRequestMessage, HttpResponseMessage> forward) : LlmRequestHandler
+    private sealed class StubHandler(Func<HttpRequestMessage, HttpResponseMessage> send) : LlmRequestHandler
     {
-        protected override Task<HttpResponseMessage> ForwardAsync(HttpRequestMessage request, LlmRequestContext ctx) =>
-            Task.FromResult(forward(request));
+        protected override Task<HttpResponseMessage> SendRequestAsync(HttpRequestMessage request, LlmRequestContext ctx) =>
+            Task.FromResult(send(request));
     }
 
-    /// <summary>A handler that adds a header in <c>TransformRequestAsync</c>.</summary>
-    private sealed class HeaderMutatingHandler(Func<HttpRequestMessage, HttpResponseMessage> forward) : LlmRequestHandler
+    /// <summary>A handler that adds a header before calling <c>base.SendRequestAsync</c>.</summary>
+    private sealed class HeaderMutatingHandler(Func<HttpRequestMessage, HttpResponseMessage> send) : LlmRequestHandler
     {
-        protected override Task<HttpRequestMessage> TransformRequestAsync(HttpRequestMessage request, LlmRequestContext ctx)
+        protected override Task<HttpResponseMessage> SendRequestAsync(HttpRequestMessage request, LlmRequestContext ctx)
         {
             request.Headers.TryAddWithoutValidation("authorization", "Bearer swapped-token");
-            return Task.FromResult(request);
+            return Task.FromResult(send(request));
         }
-
-        protected override Task<HttpResponseMessage> ForwardAsync(HttpRequestMessage request, LlmRequestContext ctx) =>
-            Task.FromResult(forward(request));
     }
 
     [Fact]

dotnet/test/Unit/LlmInference/LlmInferenceHandlerTests.cs

@@ -28,8 +28,10 @@ export {
     approveAll,
     convertMcpCallToolResult,
     createSessionFsAdapter,
+    CopilotWebSocketHandler,
+    ForwardingWebSocketHandler,
     LlmRequestHandler,
-    wrapGlobalWebSocket,
+    LlmWebSocketCloseStatus,
     SYSTEM_MESSAGE_SECTIONS,
 } from "./types.js";
 // Re-export the generated session-event types (every *Event interface and
@@ -128,7 +130,6 @@ export type {
     LlmInferenceResponseInit,
     LlmInferenceResponseSink,
     LlmRequestContext,
-    LlmWebSocketUpstream,
     SystemMessageAppendConfig,
     SystemMessageConfig,
     SystemMessageCustomizeConfig,