fix: fixes aibridge integration tests failing on windows (#24665)

Fixes aibridge responses and apidump integration tests in windows CI
job.

example failing job before the fix:
https://github.com/coder/coder/actions/runs/24873191866/job/72823942378

aibridge/internal/integrationtest/apidump_test.go

@@ -22,6 +22,8 @@ import (
 	"github.com/coder/coder/v2/aibridge/provider"
 )
 
+const osSep = string(filepath.Separator)
+
 func TestAPIDump(t *testing.T) {
 	t.Parallel()
 
@@ -164,9 +166,9 @@ func TestAPIDump(t *testing.T) {
 			require.NotEmpty(t, respDumpFile, "response dump file should exist")
 
 			// Verify dump files are in the correct provider subdirectory.
-			require.Contains(t, reqDumpFile, filepath.Join(dumpDir, tc.expectProviderDir)+"/",
+			require.Contains(t, reqDumpFile, filepath.Join(dumpDir, tc.expectProviderDir)+osSep,
 				"request dump should be in the %s provider directory", tc.expectProviderDir)
-			require.Contains(t, respDumpFile, filepath.Join(dumpDir, tc.expectProviderDir)+"/",
+			require.Contains(t, respDumpFile, filepath.Join(dumpDir, tc.expectProviderDir)+osSep,
 				"response dump should be in the %s provider directory", tc.expectProviderDir)
 
 			// Verify request dump contains expected HTTP request format.
@@ -284,12 +286,12 @@ func TestAPIDumpPassthrough(t *testing.T) {
 
 			require.NotEmpty(t, reqDumpFile, "request dump file should exist")
 			require.FileExists(t, reqDumpFile)
-			require.Contains(t, reqDumpFile, "/passthrough/")
+			require.Contains(t, reqDumpFile, osSep+"passthrough"+osSep)
 			require.Contains(t, reqDumpFile, tc.expectDumpName)
 
 			require.NotEmpty(t, respDumpFile, "response dump file should exist")
 			require.FileExists(t, respDumpFile)
-			require.Contains(t, respDumpFile, "/passthrough/")
+			require.Contains(t, respDumpFile, osSep+"passthrough"+osSep)
 			require.Contains(t, respDumpFile, tc.expectDumpName)
 
 			// Verify request dump.

aibridge/internal/integrationtest/mockupstream.go

@@ -218,18 +218,20 @@ func (ms *mockUpstream) writeSSE(w http.ResponseWriter, data []byte) {
 		return
 	}
 
-	// Write line-by-line to simulate SSE events arriving incrementally
-	scanner := bufio.NewScanner(bytes.NewReader(data))
-	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
-	for scanner.Scan() {
-		_, err := fmt.Fprintf(w, "%s\n", scanner.Text())
-		if eventstream.IsConnError(err) {
-			return // client disconnected, stop writing
+	// Write line-by-line to simulate SSE events arriving incrementally.
+	// SplitAfter keeps the line endings so fixture bytes (LF or CRLF) replay verbatim.
+	for _, line := range bytes.SplitAfter(data, []byte("\n")) {
+		if len(line) == 0 {
+			continue
+		}
+		if _, err := w.Write(line); err != nil {
+			if eventstream.IsConnError(err) {
+				return // client disconnected, stop writing
+			}
+			require.NoError(ms.t, err)
 		}
-		require.NoError(ms.t, err)
 		flusher.Flush()
 	}
-	require.NoError(ms.t, scanner.Err())
 }
 
 // isRawHTTPResponse returns true if data starts with "HTTP/", indicating

aibridge/internal/integrationtest/responses_test.go

@@ -609,13 +609,13 @@ func TestClientAndConnectionError(t *testing.T) {
 			name:        "blocking_connection_refused",
 			addr:        startRejectingListener(t),
 			streaming:   false,
-			errContains: "connection reset by peer",
+			errContains: `connection reset by peer|forcibly closed`, // RST error message differs between Linux/macOS|Windows.
 		},
 		{
 			name:        "streaming_connection_refused",
 			addr:        startRejectingListener(t),
 			streaming:   true,
-			errContains: "connection reset by peer",
+			errContains: `connection reset by peer|forcibly closed`, // RST error message differs between Linux/macOS|Windows.
 		},
 		{
 			name:        "blocking_bad_url",
@@ -1062,13 +1062,13 @@ func TestResponsesModelThoughts(t *testing.T) {
 	}
 }
 
-func requireResponsesError(t *testing.T, code int, message string, body []byte) {
+func requireResponsesError(t *testing.T, code int, messagePattern string, body []byte) {
 	var respErr responses.Error
 	err := json.Unmarshal(body, &respErr)
 	require.NoError(t, err)
 
 	require.Equal(t, strconv.Itoa(code), respErr.Code)
-	require.Contains(t, respErr.Message, message)
+	require.Regexp(t, messagePattern, respErr.Message)
 }
 
 func responsesRequestBytes(t *testing.T, streaming bool, additionalFields ...keyVal) []byte {