Update the return type hint for IntEnumField/CharEnumField from EnumType to tortoise.fields.Field[EnumType] (fixes #2155) - #2243

Summary

makemigrations generated migration files that fail mypy when a model uses CharEnumField / IntEnumField (issue #2155):

error: List item 1 has incompatible type "tuple[str, ExampleEnum]";
  expected "tuple[str, Field[Any] | ManyToManyRelation[Any] | None]"   # CreateModel
error: Argument "field" to "AddField" has incompatible type "ExampleEnum";
  expected "Field[Any] | ManyToManyRelation[Any] | None"               # AddField

The root cause is that the public IntEnumField / CharEnumField names are factory functions typed to return the enum type (a TypeVar bound to Enum) rather than a Field. That return type is intentional — it lets models write field: MyEnum = CharEnumField(MyEnum) and have the attribute resolve to the enum. But the migration writer rendered field expressions using those same factory names, so the generated fields.CharEnumField(...) call was inferred by mypy as the enum type, not a Field, and the surrounding migration operation rejected it.

Fix

Render enum fields as their concrete IntEnumFieldInstance / CharEnumFieldInstance classes instead of the factory names. These are the real Field subclasses (CharField / SmallIntField descendants), so the generated code type-checks cleanly while constructing an identical field at runtime.

• tortoise/migrations/writer.py: exclude the two enum *FieldInstance classes from the FieldInstance → Field name remapping, so they render as their concrete class.
• tortoise/fields/__init__.py: export CharEnumFieldInstance and IntEnumFieldInstance so the generated from tortoise import fields alias resolves fields.CharEnumFieldInstance(...).

Model-side typing is untouched: the factory functions still return the enum type, so field: MyEnum = CharEnumField(MyEnum) continues to type-check.

Tests / Verification

• Reproduced the issue end-to-end with the model/config from the report: tortoise --config-file config.json makemigrations then mypy migrations. Before the fix mypy emitted both reported errors; after the fix mypy migrations reports Success: no issues found for both the CreateModel (initial) and AddField cases.
• Confirmed the generated migrations still apply at runtime (tortoise ... migrate → OK).
• Updated test_writer_handles_enum_fields and added test_writer_enum_field_render_is_field_instance (a Invalid type hint for CharEnumField in generated migration script #2155 regression test covering the AddField path). Both fail on the pristine tree and pass with the fix.
• pytest tests/migrations/ (300 passed) and pytest tests/fields/ (enum + field tests, all pass).
• ruff format --check, ruff check, and mypy are clean on the changed files (no new errors vs. the pristine tree).
• Added a CHANGELOG.rst entry under 1.1.8 → Fixed.

Disclosure: prepared with AI assistance; reviewed and verified locally.