refactor: rename dispatch kwarg sync= to force_sync=

[valentijnscholten]

Summary

• in perf(watson): prefetch relations + force async indexing #14881 we introduce force_async which made me realize we should use force_sync=True instead of sync=True. IS more explicit and clear what it does.
• Rename the dispatch control kwarg sync= → force_sync= on dojo_dispatch_task / dojo_async_task / we_want_async.
• Hard rename — no alias period.

Why

• The bare name sync collides with unrelated parameters elsewhere in the codebase (e.g. JIRA_Instance.finding_jira_sync, is_keep_in_sync, scan types named "HawkScan"), making code search and review confusing.
• A recently-introduced force_async=True kwarg solves the inverse problem (always run in background). force_sync is the symmetric, intention-revealing name for the existing flag — readers instantly know what "force" means relative to the default async-when-possible behavior.

Changes

Caller / dispatch chain:

• dojo/decorators.py — we_want_async reads force_sync from kwargs.
• dojo/celery_dispatch.py — docstring.
• dojo/celery.py — DojoAsyncTask.apply_async pops force_sync (control flag, never reaches the task body).
• dojo/importers/default_importer.py, default_reimporter.py — propagate force_sync from importer kwargs to post_process_findings_batch.
• dojo/finding/helper.py — post_process_findings_batch signature + nested dojo_dispatch_task(calculate_grade, ..., force_sync=force_sync).
• dojo/api_v2/serializers.py, dojo/finding_group/views.py — JIRA push call sites that want foreground execution.

Tests:

• unittests/test_async_delete.py, test_reimport_prefetch.py, test_update_import_history.py — updated kwargs + docstrings.

Test plan

• ruff check clean on all changed files.
• pytest unittests/test_async_delete.py::TestAsyncDelete::test_async_delete_accepts_force_sync_kwarg — passes.
• Pre-existing failures in test_reimport_prefetch.py reproduce on dev baseline → not caused by this change (verified by stashing the diff and re-running).

Notes

• This PR may conflict with the watson prefetch PR (perf(watson): prefetch relations + force async indexing #14881) in dojo/decorators.py since both touch we_want_async. Whichever merges second rebases.