Trac ticket number

ticket-#36168

Branch description

This branch fixes a bug affecting backward migrations to replaced migrations in projects with squashed migrations spanning multiple apps.

The core issue was that migration_plan() mutates the global migration loader’s graph to disable replacement pruning, which impacts subsequent calls like _create_project_state(). This causes the migration graph to incorrectly include both squashed and replaced migrations as leaf nodes, leading to duplicate state mutations and errors such as FieldDoesNotExist.

The _migrate_all_backwards() method is updated to avoid relying on the mutated global loader by using a temporary loader instance with replace_migrations=False to safely generate migration plans without side effects.

This prevents global state mutation, ensures consistent project state reconstruction during backward migrations, and resolves related migration errors.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.