Update .NET Copilot SDK cookbook for current SDK releases (#3548)

* Update .NET Copilot SDK cookbook: fix On<T> breaking change, add in-process runtime, tool search, and message provenance recipes

- Fix breaking change: Session.On is now generic (On<T> where T : SessionEvent).
  Updated all recipes that used the old non-generic session.On(evt => ...) pattern:
  error-handling, pr-visualization, managing-local-files, accessibility-report,
  ralph-loop (both .md and .cs where applicable).
- Add new 'In-Process Runtime' recipe (in-process-runtime.md + recipe/in-process-runtime.cs)
  demonstrating RuntimeConnection.ForInProcess(), live-tested.
- Document tool search / deferred loading (SessionConfig.ToolSearch / ToolSearchConfig)
  in pr-visualization.md, live-tested.
- Document typed message provenance (MessageOptions.Source / MessageSource) in
  error-handling.md, live-tested.
- Fix dotnet/README.md recipe index: it was missing Ralph Loop and Accessibility
  Report entries; also added the new In-Process Runtime entry.
- Add In-Process Runtime entry to the top-level cookbook/copilot-sdk/README.md
  .NET section.
- Update dotnet/recipe/README.md runnable-examples table with the new recipe.

All existing .NET cookbook recipes were rebuilt and rechecked against the current
GitHub Copilot SDK (Copilot CLI 1.0.85); no other breaking changes were found.

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

* Update cookbook model names to current supported models

- Replace stale/nonexistent model IDs (gpt-5, claude-sonnet-4.5,
  claude-opus-4.6, gpt-5.1-codex-mini) with current values from the
  supported AI models list
- Prefer Model = "auto" wherever a specific model isn't load-bearing
  to the recipe's teaching point
- ralph-loop: pin gpt-5.3-codex (current codex-family GA model), since
  a dedicated coding model is a deliberate, documented choice for the
  autonomous loop
- multiple-sessions: reshape the model narrative so it demonstrates a
  real explicit-override use case (A/B testing via claude-sonnet-5)
  instead of two sessions sharing the same hardcoded model

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

* Use SendAndWaitAsync in in-process runtime recipe

Replace the event-based TaskCompletionSource pattern with SendAndWaitAsync so the example waits for the final assistant response, propagates session errors, and uses the SDK timeout behavior.

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

---------

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

cookbook/copilot-sdk/dotnet/recipe/multiple-sessions.cs

@@ -7,20 +7,22 @@ using GitHub.Copilot;
 await using var client = new CopilotClient();
 await client.StartAsync();
 
-// Create multiple independent sessions
+// Create multiple independent sessions. Most sessions should let Copilot pick the
+// best model automatically; pin an explicit model only when you have a deliberate
+// reason to (e.g. an A/B comparison, as with session3 here).
 var session1 = await client.CreateSessionAsync(new SessionConfig
 {
-    Model = "gpt-5",
+    Model = "auto",
     OnPermissionRequest = PermissionHandler.ApproveAll
 });
 var session2 = await client.CreateSessionAsync(new SessionConfig
 {
-    Model = "gpt-5",
+    Model = "auto",
     OnPermissionRequest = PermissionHandler.ApproveAll
 });
 var session3 = await client.CreateSessionAsync(new SessionConfig
 {
-    Model = "claude-sonnet-4.5",
+    Model = "claude-sonnet-5",
     OnPermissionRequest = PermissionHandler.ApproveAll
 });