[wrangler] serialize custom build watch events to avoid concurrent runs

[dhruv7539]

Summary

• serialize custom build watch-triggered runs in BundlerController
• keep only the latest pending file-change build request when many events arrive in a burst
• preserve stale-build suppression by retaining the existing abort-controller checks

Why

Fixes the behavior in #10944 where rapid multi-file changes can start multiple custom builds at once.

Testing

• pnpm -C packages/wrangler test src/__tests__/api/startDevWorker/BundleController.test.ts
• pnpm -C packages/wrangler exec eslint src/api/startDevWorker/BundlerController.ts src/__tests__/api/startDevWorker/BundleController.test.ts

Added regression test

• custom build watcher changes are processed without concurrent custom builds
• uses a lock file in a test custom-build script to fail if two builds overlap
• updates multiple watched files in quick succession and verifies no Custom build failed events are emitted

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8ae0df1

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[dhruv7539]

Addressed both actionable comments in the latest push (8ae0df1):\n- fixed wiring in custom-build path to use \n- added a Wrangler patch changeset ()

[dhruv7539]

Addressed both actionable comments in the latest push (8ae0df1ef):

• fixed jsxFragment wiring in BundlerController custom-build path to use config.build.jsxFragment
• added a Wrangler patch changeset (.changeset/quick-keys-smell.md)