feat: support on-demand retrieval tasks without affecting RSR scoring

[Hany-Almnaem]

Summary

This PR adds support for on-demand retrieval tasks in Spark Checker, allowing storage providers (SPs) or developers to request ad-hoc checks for debugging or configuration validation — without affecting RSR scoring.

Implements: #103 – Allow SP to request more frequent checks

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What’s Included

Core Logic

• tasker.js

  • Added queueOnDemandTask() and a private #onDemandTasks queue
  • Updated next() to prioritize on-demand tasks
  • Tagged tasks with isOnDemand: true

• spark.js

  • submitMeasurement() includes excludeFromRSR: true when submitting on-demand tasks

Test

• tasker.test.js: Verifies that on-demand tasks are returned before round tasks
• spark.test.js: Confirms excludeFromRSR is included in the measurement payload when isOnDemand is set

Manual Testing Script

• scripts/trigger-on-demand.js
  • Demonstrates how to queue an on-demand task manually

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Notes

• No HTTP endpoint was added. This should be considered separately to ensure secure access and abuse prevention.
• All changes are backward-compatible

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Let me know if any adjustments are needed .