mypy(workflow-engine): Prepare for detector.project to be nullable

[leeandher]

sorry for all the codeowners pings 🙏

we're looking to support a new detector for All Projects, but the current model (Detector) has a FK to Project. We can't use a random project without having issues with things like cascade deletions and project transfers so we're opting to make the column nullable.

That will introduce a lot of mypy issues though, so before that migration we're addressing those here.

Tech Spec - https://app.notion.com/p/sentry/Spec-All-Projects-Default-Alert-Migration-8e48b10e4b5d821fb1c581cc65e65c32?source=copy_link
Linear Ticket - https://linear.app/getsentry/issue/ISWF-3123/create-migration-to-support-all-projects

[linear-code]

ISWF-3123

[saponifi3d]

lgtm - just an unrelated note to keep in mind for the future. i also confirmed that there shouldn't be any other call sites. 🚢

[sentry-warden]

Cache invalidation crashes for non-project-scoped detectors

get_org_from_detector now uses detector.linked_project, which raises ValueError when project is None. This function is registered as a post_save/post_delete signal handler for all Detector models, so saving or deleting any non-project-scoped detector (introduced by this PR) will propagate an unhandled exception through Django signals.

Evidence

• Detector.linked_project raises ValueError("Detector is not project-scoped") when self.project is None (src/sentry/workflow_engine/models/detector.py:123-124).
• @cache_func_for_models([(Detector, get_org_from_detector)]) registers get_org_from_detector as the arg_getter for post_save and post_delete signals on every Detector instance (src/sentry/utils/function_cache.py:66-97).
• clear_cache_for_cached_func calls arg_getter(instance) without any try/except (src/sentry/utils/function_cache.py:47), so the ValueError propagates directly.
• The PR explicitly introduces detectors with nullable project ("All Projects"), meaning uptime code will now hit this crash path whenever one of those detectors is saved or deleted.
• The cached function get_active_auto_monitor_count_for_org only cares about uptime detectors with project__organization, so a project=None detector is irrelevant to its cache, but the signal still fires unconditionally.

_{Identified by Warden · sentry-backend-bugs · 3CL-M2W}