RFC: Fixes #20290: Prevent ObjectType queries when table doesn't exist

[jnovinger]

Fixes: #20290

In v4.4.0, ObjectType became a concrete model with a database table. Signal handlers query this table during migrations, but the table doesn't exist yet during 3.7.x→4.4.0 upgrades. The query failures poison the transaction and abort migrations.

This commits adds a objecttype_table_exists() helper and and uses it in three places:

• has_feature() in netbox/models/features.py
• update_object_types() in core/signals.py
• Search backend cache() in search/backends.py

If core_objecttype table doesn't exist, operations return early instead of querying. Once table exists, normal operation resumes.

Pros

• no migration changes
• one helper function, three identical checks
• easy to test, verify, and remove later if needed
• queries PostgreSQL system catalogs for table existence - won't poison transactions

Cons

• fixes symptoms, not root cause
• touches 3 different files instead of one surgical fix
• future code querying ObjectType during migrations could still fail/future developers might not know to add this check in new code
• adds table introspection overhead (could cache if needed) to every has_feature() call
• could introspect table name from model instead of hard-coding

[jnovinger]

Other approaches considered:

1. Squashed migrations with updated dependencies (per @pheus's comment in Upgrade from v3.7.8 to v4.4.0 fails on migration users.0005_alter_user_table (core_objecttype missing) #20290) - Changes already-applied migrations, breaks 4.3.7 -> 4.4.0 upgrades
2. Earlier ObjectType table creation via dependency restructuring - Same issues as squashing, multiple upgrade paths make this impractical
3. Global signal disconnection during migrations - Too broad, loses legitimate functionality
4. Dual feature detection (DB + in-memory fallback) - Re-adds code we just removed in v4.4.0
5. Lazy ObjectType evaluation - Doesn't address direct ObjectType queries in core/signals.py and search/backends.py
6. Document intermediate upgrade step requirement - Still viable, could complement this fix