Use in-memory tracking for per-handler maxParallelTasks and forward parallel option

[vkarpov15]

Motivation

• Avoid expensive and racy database countDocuments() calls when enforcing per-handler concurrency limits and instead track running counts in memory during polling.
• Ensure worker-level concurrency configured via startPolling({ parallel }) is honored by poll() so the worker can limit overall parallelism.
• Allow handlers to express a per-handler concurrency cap so a single task type cannot monopolize a worker.

Description

• Update registerHandler to accept an options parameter and store handlers as { handler, maxParallelTasks } so maxParallelTasks is available to the poll loop.
• Change startPolling to forward the parallel option into poll() via this._currentPoll so poll() knows worker-level concurrency.
• Replace DB counting in poll() with an in-memory handlerLimits map and a runningByName map, and filter out blocked names using filter.name = { $nin: blockedNames } before findOneAndUpdate, while incrementing/decrementing runningByName around each execute() call.
• Update execute() to resolve the stored handler entry shape and call the handler function via the resolved handlerFn (preserving timeouts and repeat behavior).

Testing

• Added unit tests in test/task.test.js including a waitFor helper and tests for forwarding parallel to startPolling() and for enforcing maxParallelTasks per handler.
• Automated tests were not run in this environment because the test suite depends on a running MongoDB instance and timed out during setup, so no test results are available here.

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Codex Task

[Copilot]

Pull request overview

This PR implements in-memory tracking for per-handler concurrency limits to replace expensive database counting operations. It allows workers to enforce both worker-level and per-handler concurrency limits simultaneously.

Changes:

• Added in-memory tracking via handlerLimits and runningByName maps in the poll() method to efficiently enforce per-handler concurrency without database queries
• Extended registerHandler() to accept an optional options parameter with maxParallelTasks to specify per-handler concurrency limits
• Modified startPolling() to forward the parallel option to poll() for worker-level concurrency control
• Updated execute() to handle both the new object-based handler format and legacy function-based format for backward compatibility

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/taskSchema.js	Core implementation of in-memory concurrency tracking, handler registration changes, and backward-compatible handler resolution
test/task.test.js	Added test utilities (waitFor) and comprehensive tests for parallel option forwarding and per-handler concurrency limits
README.md	Added documentation explaining the new parallelism features with clear code examples

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