Feat: Add optional per-task context propagation

[mday-io]

Description

Add optional context_factory support to the concurrency helpers. Capture context once per invocation and give each task its own copy, including serial execution. Omitting the factory preserves existing behavior.

#6140 is merged. This PR now contains only context propagation and is a prerequisite for #6051.

Test Plan

• Rebased onto main 3891d65a7; git range-diff confirms the feature patch is unchanged.
• All 17 concurrency tests pass, including context isolation, caller context, executor reuse and terminal worker failures.
• Snapshot-wrapper probe passes with one and three workers; context reaches callbacks without leaking their changes to siblings or the caller.
• Independent review found no correctness issues.
• make style and all six make fast-test batches pass, with 2,895 tests passed and 13 skipped.
• Confirm new-head CI, including Windows.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO