Gate vast allocation create/activate/deactivate on directory presence

[claire-peters]

Summary

Follow-up to #487 (merged). userquotas alone isn't authoritative about whether a project's directory actually exists on VAST — it's just quota configuration, and PR #487's identity-model fix still risked creating/activating allocations, or leaving stale ones active, based on quota-list membership alone. This adds folders.stat_path as ground truth, per the real VAST API docs the user found:

• get_vast_directory_stat(path) wraps client.folders.stat_path.post(path=...). Returns the stat dict when present, None on VAST's 503 "not found" response, and re-raises any other failure — an auth/network error must never be mistaken for "directory doesn't exist," since that risks repeating the same mass-wrongful-deactivation bug PR Add sync_vast_allocations management command #487 fixed.
• sync_allocation_for_vast_quota now only creates a new allocation or activates a pending request when the project's directory (/{resource_url}/C/{project.title}) is confirmed present; otherwise it's skipped and recorded in a new report['directory_missing'] list.
• deactivate_allocations_with_missing_directory replaces the userquotas-list-membership check: it stat_path-checks every Active allocation on the resource directly, regardless of whether its project appeared in this run's userquotas at all — catches a project that's vanished from the quota list entirely, not just one whose directory is empty.

Test plan

• manage.py test coldfront.plugins.vast — 26 tests pass (mocked; no live VAST cluster needed)
• manage.py check — clean
• Dry-run sync_vast_allocations against real VAST data in a non-prod environment and confirm report['directory_missing'] and report['deactivated'] match expectations before relying on it in production