Update test to expect improved Response.Flush error message

The test TestResponse_FlushPanics was expecting the old generic error
message but should now expect the improved message that includes the
specific ResponseWriter type information.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

response_test.go

@@ -80,7 +80,7 @@ func TestResponse_FlushPanics(t *testing.T) {
 	res := &Response{echo: e, Writer: rw}
 
 	// we test that we behave as before unwrapping flushers - flushing writer that does not support it causes panic
-	assert.PanicsWithError(t, "response writer flushing is not supported", func() {
+	assert.PanicsWithError(t, "echo: response writer *echo.testResponseWriter does not support flushing (http.Flusher interface)", func() {
 		res.Flush()
 	})
 }