fix(mcp): bring trace bundles under output budget accounting

Per review on #41021. The previous "We don't own writing so we cannot
track file size for this" comment meant traces were pinned outside the
budget, so a single trace could blow the cap unboundedly.

Drop `evictable: false` from the traces directory and, on tracingStop,
walk the produced trace files via OutputDir.trackTree to register their
sizes. Old traces become evictable like any other artifact.

We still can't account for the trace mid-recording (no hooks), but
post-stop accounting is enough to keep multi-trace sessions under cap.

Author: Skn0tt

packages/playwright-core/src/tools/backend/outputDir.ts

@@ -70,6 +70,28 @@ export class OutputDir {
     }
   }
 
+  // Register every file under `dir` with its current size on disk. Useful for
+  // bringing externally-written files (e.g., trace bundles) under accounting.
+  async trackTree(dir: string): Promise<void> {
+    if (this.maxSize === 0)
+      return;
+    let entries: fs.Dirent[];
+    try {
+      entries = await fs.promises.readdir(dir, { withFileTypes: true });
+    } catch {
+      return;
+    }
+    for (const entry of entries) {
+      const full = path.join(dir, entry.name);
+      if (entry.isDirectory()) {
+        await this.trackTree(full);
+      } else if (entry.isFile()) {
+        const file = await this.resolve(full);
+        await file.trackSize();
+      }
+    }
+  }
+
 }
 
 export class OutputFile {

packages/playwright-core/src/tools/backend/tracing.ts

@@ -31,8 +31,7 @@ const tracingStart = defineTool({
 
   handle: async (context, params, response) => {
     const browserContext = await context.ensureBrowserContext();
-    // We don't own writing so we cannot track file size for this.
-    const { path: tracesDir } = await context.outputFile({ prefix: '', suggestedFilename: `traces`, ext: '' }, { origin: 'code', evictable: false });
+    const { path: tracesDir } = await context.outputFile({ prefix: '', suggestedFilename: `traces`, ext: '' }, { origin: 'code' });
     const name = 'trace-' + Date.now();
     await browserContext.tracing.start({
       name,
@@ -70,6 +69,9 @@ const tracingStop = defineTool({
     // eslint-disable-next-line no-restricted-syntax
     delete (browserContext.tracing as any)[traceLegendSymbol];
 
+    // Now that the trace bundle is on disk, bring it under budget accounting.
+    await context.outputDir.trackTree(traceLegend.tracesDir);
+
     response.addTextResult(`Trace recording stopped.`);
     response.addFileLink('Trace', `${traceLegend.tracesDir}/${traceLegend.name}.trace`);
     response.addFileLink('Network log', `${traceLegend.tracesDir}/${traceLegend.name}.network`);