Feat: Add optional per-task context propagation

[mday-io]

Summary

Add optional context_factory support so concurrent tasks can receive caller context without sharing each other’s changes.

• Capture context once per run or batch and copy it for each task.
• Preserve existing behavior when no factory is supplied.
• Depends on Fix: Propagate terminal worker failures and stop queued DAG work #6140; prerequisite for Feat: Add optional execution observations #6051. The current diff includes Fix: Propagate terminal worker failures and stop queued DAG work #6140.

Test Plan

• Four context tests failed against the old API and passed with the new argument.
• Tested serial and threaded isolation, caller context and executor reuse.
• Checked default DAG behavior and snapshot-wrapper forwarding.
• Ran make style and all make fast-test batches: 2,866 passed.
• Confirm current-head CI, including Windows.