Fix data race in TestStreamingFidelity

Add sync.Mutex to protect concurrent access to the events slice
in all three subtests, matching the pattern used throughout the
rest of the E2E test suite.

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

Author: stephentoub

go/internal/e2e/streaming_fidelity_test.go

@@ -2,6 +2,7 @@ package e2e
 
 import (
 	"strings"
+	"sync"
 	"testing"
 
 	copilot "github.com/github/copilot-sdk/go"
@@ -25,18 +26,26 @@ func TestStreamingFidelity(t *testing.T) {
 		}
 
 		var events []copilot.SessionEvent
+		var mu sync.Mutex
 		session.On(func(event copilot.SessionEvent) {
+			mu.Lock()
 			events = append(events, event)
+			mu.Unlock()
 		})
 
 		_, err = session.SendAndWait(t.Context(), copilot.MessageOptions{Prompt: "Count from 1 to 5, separated by commas."})
 		if err != nil {
 			t.Fatalf("Failed to send message: %v", err)
 		}
 
+		mu.Lock()
+		snapshot := make([]copilot.SessionEvent, len(events))
+		copy(snapshot, events)
+		mu.Unlock()
+
 		// Should have streaming deltas before the final message
 		var deltaEvents []copilot.SessionEvent
-		for _, e := range events {
+		for _, e := range snapshot {
 			if e.Type == "assistant.message_delta" {
 				deltaEvents = append(deltaEvents, e)
 			}
@@ -54,7 +63,7 @@ func TestStreamingFidelity(t *testing.T) {
 
 		// Should still have a final assistant.message
 		hasAssistantMessage := false
-		for _, e := range events {
+		for _, e := range snapshot {
 			if e.Type == "assistant.message" {
 				hasAssistantMessage = true
 				break
@@ -67,7 +76,7 @@ func TestStreamingFidelity(t *testing.T) {
 		// Deltas should come before the final message
 		firstDeltaIdx := -1
 		lastAssistantIdx := -1
-		for i, e := range events {
+		for i, e := range snapshot {
 			if e.Type == "assistant.message_delta" && firstDeltaIdx == -1 {
 				firstDeltaIdx = i
 			}
@@ -92,18 +101,26 @@ func TestStreamingFidelity(t *testing.T) {
 		}
 
 		var events []copilot.SessionEvent
+		var mu sync.Mutex
 		session.On(func(event copilot.SessionEvent) {
+			mu.Lock()
 			events = append(events, event)
+			mu.Unlock()
 		})
 
 		_, err = session.SendAndWait(t.Context(), copilot.MessageOptions{Prompt: "Say 'hello world'."})
 		if err != nil {
 			t.Fatalf("Failed to send message: %v", err)
 		}
 
+		mu.Lock()
+		snapshot := make([]copilot.SessionEvent, len(events))
+		copy(snapshot, events)
+		mu.Unlock()
+
 		// No deltas when streaming is off
 		var deltaEvents []copilot.SessionEvent
-		for _, e := range events {
+		for _, e := range snapshot {
 			if e.Type == "assistant.message_delta" {
 				deltaEvents = append(deltaEvents, e)
 			}
@@ -114,7 +131,7 @@ func TestStreamingFidelity(t *testing.T) {
 
 		// But should still have a final assistant.message
 		var assistantEvents []copilot.SessionEvent
-		for _, e := range events {
+		for _, e := range snapshot {
 			if e.Type == "assistant.message" {
 				assistantEvents = append(assistantEvents, e)
 			}
@@ -153,8 +170,11 @@ func TestStreamingFidelity(t *testing.T) {
 		}
 
 		var events []copilot.SessionEvent
+		var mu sync.Mutex
 		session2.On(func(event copilot.SessionEvent) {
+			mu.Lock()
 			events = append(events, event)
+			mu.Unlock()
 		})
 
 		answer, err := session2.SendAndWait(t.Context(), copilot.MessageOptions{Prompt: "Now if you double that, what do you get?"})
@@ -167,9 +187,14 @@ func TestStreamingFidelity(t *testing.T) {
 			t.Errorf("Expected answer to contain '18', got %v", answer)
 		}
 
+		mu.Lock()
+		snapshot := make([]copilot.SessionEvent, len(events))
+		copy(snapshot, events)
+		mu.Unlock()
+
 		// Should have streaming deltas before the final message
 		var deltaEvents []copilot.SessionEvent
-		for _, e := range events {
+		for _, e := range snapshot {
 			if e.Type == "assistant.message_delta" {
 				deltaEvents = append(deltaEvents, e)
 			}