fix(task): preserve caller tx on cancel

[imbajin]

Purpose of the PR

Cancelling a task can close the caller thread's existing graph transaction and discard unrelated, uncommitted graph changes. The default completion callback can do the same when cancellation invokes it on the caller thread.

This independently fixes the caller transaction issue tracked in the loading-mode review.

Main Changes

Preserve an existing caller transaction during Standard and Distributed scheduler cancellation, and let cancelled tasks leave worker cleanup to their execution path. Keep normal worker completion and task ownership fencing in place.

Before: write uncommitted data → cancel another task → that data is rolled back.
After: write uncommitted data → cancel another task → the caller can still commit or roll back its own data.

Document transaction ownership in docs/server-runtime.md. Update the missing-retry test fixture to provide the transaction interface that cancellation now inspects.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests.
• Need tests and can be verified as follows:
  • Real graph transactions reproduce the loss with the original code and preserve caller data after queued, running, completed, direct and failed cancellation. Separate worker controls verify cleanup and owner fencing.
  • Relevant tests, formatting and root compilation passed. The full unit suite passed with the canonical snapshot test fixture from #3339; production classes were unchanged by that test-only overlay, and existing environment assumptions remain.
  • Three independent reviewers checked the final change. Current org/ASF CI will be verified on the submitted head.

Does this PR potentially affect the following parts?

• Dependencies (add/update license info)
• Modify configurations
• The public API
• Other affects: task cancellation and thread-local transaction ownership
• Nope

Documentation Status

• Doc - TODO: required documentation is pending; complete it before merging.
• Doc - Done: documentation is included here or linked below.
• Doc - No Need: no user-visible documentation is affected.

Documentation files in this PR or paired hugegraph-doc PR: docs/server-runtime.md.

Paired source PR: hugegraph#311

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.01%. Comparing base (92af2f8) to head (0bff7ee).

Files with missing lines	Patch %	Lines
...pache/hugegraph/task/DistributedTaskScheduler.java	70.00%	3 Missing ⚠️
...n/java/org/apache/hugegraph/task/TaskCallable.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3340      +/-   ##
============================================
+ Coverage     54.31%   57.01%   +2.70%     
+ Complexity     2250      499    -1751     
============================================
  Files           770      514     -256     
  Lines         67952    49740   -18212     
  Branches       9213     7206    -2007     
============================================
- Hits          36905    28360    -8545     
+ Misses        27470    18480    -8990     
+ Partials       3577     2900     -677     

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