fix(websocket): add missing incPendingActivityCount() in blob binaryType case (#26670)

## Summary

- Fix crash ("Pure virtual function called!") when WebSocket client
receives binary data with `binaryType = "blob"` and no event listener
attached
- Add missing `incPendingActivityCount()` call before `postTask` in the
Blob case of `didReceiveBinaryData`
- Add regression test for issue #26669

## Root Cause

The Blob case in `didReceiveBinaryData` (WebSocket.cpp:1324-1331) was
calling `decPendingActivityCount()` inside the `postTask` callback
without a matching `incPendingActivityCount()` beforehand. This bug was
introduced in #21471 when Blob support was added.

The ArrayBuffer and NodeBuffer cases correctly call
`incPendingActivityCount()` before `postTask`, but the Blob case was
missing this call.

## Test plan

- [x] New regression test verifies WebSocket with `binaryType = "blob"`
doesn't crash on ping frames
- [x] `bun bd test test/regression/issue/26669.test.ts` passes

Fixes #26669

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

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>
Co-authored-by: Ciro Spaciari MacBook <ciro@anthropic.com>

src/bun.js/bindings/webcore/WebSocket.cpp

@@ -1323,6 +1323,7 @@ void WebSocket::didReceiveBinaryData(const AtomString& eventName, const std::spa
 
         if (auto* context = scriptExecutionContext()) {
             RefPtr<Blob> blob = Blob::create(binaryData, context->jsGlobalObject());
+            this->incPendingActivityCount();
             context->postTask([this, name = eventName, blob = blob.releaseNonNull(), protectedThis = Ref { *this }](ScriptExecutionContext& context) {
                 ASSERT(scriptExecutionContext());
                 protectedThis->dispatchEvent(MessageEvent::create(name, blob, protectedThis->m_url.string()));

test/regression/issue/26669.test.ts

@@ -0,0 +1,69 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+// https://github.com/oven-sh/bun/issues/26669
+// WebSocket client crashes ("Pure virtual function called!") when binaryType = "blob"
+// and no event listener is attached. The missing incPendingActivityCount() allows the
+// WebSocket to be GC'd before the postTask callback runs.
+test("WebSocket with binaryType blob should not crash when GC'd before postTask", async () => {
+  await using server = Bun.serve({
+    port: 0,
+    fetch(req, server) {
+      if (server.upgrade(req)) return undefined;
+      return new Response("Not a websocket");
+    },
+    websocket: {
+      open(ws) {
+        // Send binary data immediately - this triggers didReceiveBinaryData
+        // with the Blob path when client has binaryType = "blob"
+        ws.sendBinary(new Uint8Array(64));
+        ws.sendBinary(new Uint8Array(64));
+        ws.sendBinary(new Uint8Array(64));
+      },
+      message() {},
+    },
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [
+      bunExe(),
+      "-e",
+      `
+const url = process.argv[1];
+// Create many short-lived WebSocket objects with blob binaryType and no listeners.
+// Without the fix, the missing incPendingActivityCount() lets the WebSocket get GC'd
+// before the postTask callback fires, causing "Pure virtual function called!".
+async function run() {
+  for (let i = 0; i < 100; i++) {
+    const ws = new WebSocket(url);
+    ws.binaryType = "blob";
+    // Intentionally: NO event listeners attached.
+    // This forces the postTask path in didReceiveBinaryData's Blob case.
+  }
+  // Force GC to collect the unreferenced WebSocket objects while postTask
+  // callbacks are still pending.
+  Bun.gc(true);
+  await Bun.sleep(50);
+  Bun.gc(true);
+  await Bun.sleep(50);
+  Bun.gc(true);
+  await Bun.sleep(100);
+}
+await run();
+Bun.gc(true);
+await Bun.sleep(200);
+console.log("OK");
+process.exit(0);
+`,
+      `ws://localhost:${server.port}`,
+    ],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stdout).toContain("OK");
+  expect(exitCode).toBe(0);
+});