Add maxWorkflowThreadHeapMiB to prevent OOM crashes from workflow cache

[sachinsharma3191]

Summary

Adds a new WorkerOptions.maxWorkflowThreadHeapMiB option that sets resourceLimits.maxOldGenerationSizeMb on workflow worker threads created in threaded-vm.ts.

Problem: The workflow cache is count-based LRU (maxCachedWorkflows) with no memory awareness. When per-workflow memory footprint is large (e.g. heavy zod schemas or top-level module state), the cache can exhaust the V8 heap, causing ERR_WORKER_OUT_OF_MEMORY — a fatal, unrecoverable crash that kills the entire worker process.

Solution: By setting resourceLimits on the node:worker_threads Worker constructor, V8 heap exhaustion in a workflow thread becomes a catchable error instead of a process crash. The existing UnexpectedError handling path already reports the failure and initiates graceful shutdown.

Changes

• worker-options.ts: New maxWorkflowThreadHeapMiB?: number option with documentation. Validation rejects non-positive values.
• worker.ts: Passes the option through to ThreadedVMWorkflowCreator.create().
• threaded-vm.ts:
  • Accepts maxOldGenerationSizeMb and passes it as resourceLimits to new NodeWorker()
  • Detects OOM errors specifically and logs an actionable message suggesting maxCachedWorkflows or maxWorkflowThreadHeapMiB tuning

Usage

const worker = await Worker.create({
  taskQueue: 'my-task-queue',
  workflowsPath: require.resolve('./workflows'),
  maxWorkflowThreadHeapMiB: 512, // cap each thread to 512 MiB
});

Design note

This is the minimal, self-contained fix that prevents the fatal crash. A full memory-aware cache (feeding V8 heap stats back to the Rust core's eviction decisions) would require cross-layer changes and is tracked separately. This PR gives users an immediate escape hatch.

Ref #2227

Test plan

• Syntax and structural correctness verified (TypeScript parses clean)
• resourceLimits is only set when maxWorkflowThreadHeapMiB is provided (default behavior unchanged)
• Validation: maxWorkflowThreadHeapMiB <= 0 throws TypeError
• OOM error detection: specific log message with tuning guidance
• Manual: Reproduce OOM with millerick's repro, verify worker survives with maxWorkflowThreadHeapMiB set