Fix CAPI reasoning E2E fixtures

Use gpt-5.4 for tests that explicitly configure high reasoning effort, and advertise that model in the streaming replay fixture.

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

Copilot-Session: 9b80f6cb-b851-4598-8294-f4431c6f3143

Author: ellismg

dotnet/test/E2E/SessionE2ETests.cs

@@ -615,10 +615,10 @@ public async Task Should_Set_Model_With_ReasoningEffort()
 
         var modelChangedTask = TestHelper.GetNextEventOfTypeAsync<SessionModelChangeEvent>(session);
 
-        await session.SetModelAsync("gpt-4.1", "high");
+        await session.SetModelAsync("gpt-5.4", "high");
 
         var modelChanged = await modelChangedTask;
-        Assert.Equal("gpt-4.1", modelChanged.Data.NewModel);
+        Assert.Equal("gpt-5.4", modelChanged.Data.NewModel);
         Assert.Equal("high", modelChanged.Data.ReasoningEffort);
     }
 

dotnet/test/E2E/StreamingFidelityE2ETests.cs

@@ -152,6 +152,7 @@ public async Task Should_Emit_Streaming_Deltas_With_Reasoning_Effort_Configured(
         // the streaming pipeline — deltas still arrive and complete successfully.
         var session = await CreateSessionAsync(new SessionConfig
         {
+            Model = "gpt-5.4",
             Streaming = true,
             ReasoningEffort = "high",
         });

test/snapshots/streaming_fidelity/should_emit_streaming_deltas_with_reasoning_effort_configured.yaml

@@ -1,5 +1,5 @@
 models:
-  - claude-sonnet-4.5
+  - gpt-5.4
 conversations:
   - messages:
       - role: system