fix: round 2 of CI fixes for expanded E2E coverage

- StreamingFidelityE2ETests: lock around the events list in the three
  remaining streaming tests. Without locking, the event handler runs on
  the dispatcher thread and continues to append after SendAndWaitAsync
  returns, racing with the test thread's enumeration and producing
  "Collection was modified; enumeration operation may not execute".
  This mirrors the lock-and-snapshot pattern already used in the
  Should_Emit_AssistantMessageStart_Before_Deltas_With_Matching_MessageId
  test in the same file.
- python/pyproject.toml: add opentelemetry-sdk to dev dependencies so
  test_telemetry_e2e.py::test_restores_w3c_trace_context can construct
  a real TracerProvider. The opentelemetry-api package alone provides
  the namespace package "opentelemetry" but not "opentelemetry.sdk".

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

Author: stephentoub

dotnet/test/E2E/StreamingFidelityE2ETests.cs

@@ -16,14 +16,17 @@ public async Task Should_Produce_Delta_Events_When_Streaming_Is_Enabled()
         var session = await CreateSessionAsync(new SessionConfig { Streaming = true });
 
         var events = new List<SessionEvent>();
-        session.On(evt => events.Add(evt));
+        session.On(evt => { lock (events) { events.Add(evt); } });
 
         await session.SendAndWaitAsync(new MessageOptions { Prompt = "Count from 1 to 5, separated by commas." });
 
-        var types = events.Select(e => e.Type).ToList();
+        List<SessionEvent> snapshot;
+        lock (events) { snapshot = [.. events]; }
+
+        var types = snapshot.Select(e => e.Type).ToList();
 
         // Should have streaming deltas before the final message
-        var deltaEvents = events.OfType<AssistantMessageDeltaEvent>().ToList();
+        var deltaEvents = snapshot.OfType<AssistantMessageDeltaEvent>().ToList();
         Assert.NotEmpty(deltaEvents);
 
         // Deltas should have content
@@ -49,17 +52,20 @@ public async Task Should_Not_Produce_Deltas_When_Streaming_Is_Disabled()
         var session = await CreateSessionAsync(new SessionConfig { Streaming = false });
 
         var events = new List<SessionEvent>();
-        session.On(evt => events.Add(evt));
+        session.On(evt => { lock (events) { events.Add(evt); } });
 
         await session.SendAndWaitAsync(new MessageOptions { Prompt = "Say 'hello world'." });
 
-        var deltaEvents = events.OfType<AssistantMessageDeltaEvent>().ToList();
+        List<SessionEvent> snapshot;
+        lock (events) { snapshot = [.. events]; }
+
+        var deltaEvents = snapshot.OfType<AssistantMessageDeltaEvent>().ToList();
 
         // No deltas when streaming is off
         Assert.Empty(deltaEvents);
 
         // But should still have a final assistant.message
-        var assistantEvents = events.OfType<AssistantMessageEvent>().ToList();
+        var assistantEvents = snapshot.OfType<AssistantMessageEvent>().ToList();
         Assert.NotEmpty(assistantEvents);
 
         await session.DisposeAsync();
@@ -78,14 +84,17 @@ public async Task Should_Produce_Deltas_After_Session_Resume()
             new ResumeSessionConfig { OnPermissionRequest = PermissionHandler.ApproveAll, Streaming = true });
 
         var events = new List<SessionEvent>();
-        session2.On(evt => events.Add(evt));
+        session2.On(evt => { lock (events) { events.Add(evt); } });
 
         var answer = await session2.SendAndWaitAsync(new MessageOptions { Prompt = "Now if you double that, what do you get?" });
         Assert.NotNull(answer);
         Assert.Contains("18", answer!.Data.Content ?? string.Empty);
 
+        List<SessionEvent> snapshot;
+        lock (events) { snapshot = [.. events]; }
+
         // Should have streaming deltas before the final message
-        var deltaEvents = events.OfType<AssistantMessageDeltaEvent>().ToList();
+        var deltaEvents = snapshot.OfType<AssistantMessageDeltaEvent>().ToList();
         Assert.NotEmpty(deltaEvents);
 
         // Deltas should have content

python/pyproject.toml

@@ -39,6 +39,7 @@ dev = [
     "pytest-asyncio>=0.21.0",
     "pytest-timeout>=2.0.0",
     "httpx>=0.24.0",
+    "opentelemetry-sdk>=1.0.0",
 ]
 telemetry = [
     "opentelemetry-api>=1.0.0",