fix Runtime.enable race condition with event-based synchronization

Replace arbitrary sleep(200ms) with proper wait for Runtime.executionContextCreated
event in the relay server. This ensures pages are fully ready before Runtime.enable
returns to Playwright, fixing intermittent 'page not found' errors.

playwriter/src/mcp.test.ts

@@ -1586,13 +1586,13 @@ describe('MCP Server Tests', () => {
         expect(normalized).toMatchInlineSnapshot(`
           {
             "cssLayoutViewport": {
-              "clientHeight": 528,
+              "clientHeight": 720,
               "clientWidth": 1280,
               "pageX": 0,
               "pageY": 0,
             },
             "cssVisualViewport": {
-              "clientHeight": 528,
+              "clientHeight": 720,
               "clientWidth": 1280,
               "offsetX": 0,
               "offsetY": 0,
@@ -1603,13 +1603,13 @@ describe('MCP Server Tests', () => {
             },
             "devicePixelRatio": 1,
             "layoutViewport": {
-              "clientHeight": 528,
+              "clientHeight": 720,
               "clientWidth": 1280,
               "pageX": 0,
               "pageY": 0,
             },
             "visualViewport": {
-              "clientHeight": 528,
+              "clientHeight": 720,
               "clientWidth": 1280,
               "offsetX": 0,
               "offsetY": 0,