feat: deprecate implicit on_delete default in ForeignKeyField, OneToOneField, ManyToManyField

[Yad4o]

Summary

Closes #1801.

ForeignKeyField, OneToOneField, and ManyToManyField silently defaulted on_delete to CASCADE. This is dangerous — a developer who forgets to set on_delete may inadvertently cascade-delete data they didn't intend to.

This PR adds a DeprecationWarning when on_delete is omitted, making the implicit behaviour visible and encouraging explicit intent. The default behaviour is preserved for now to avoid a breaking change.

Changes

• Added _UNSET sentinel in tortoise/fields/relational.py to detect when on_delete is not passed
• ForeignKeyFieldInstance.__init__: emits DeprecationWarning when on_delete is not explicitly provided
• OneToOneFieldInstance.__init__: same (delegates to ForeignKeyFieldInstance)
• ManyToManyFieldInstance.__init__: same
• All three public factory functions (ForeignKeyField, OneToOneField, ManyToManyField) updated to use the sentinel as default
• @overload signatures are unchanged (type checkers still see OnDelete = CASCADE) so no type-checking regressions

Before / After

# Before — silent CASCADE, no indication this is dangerous
class MyModel(Model):
    user = fields.ForeignKeyField("models.User")  # silently cascades on delete

# After — explicit warning shown at import/class-definition time
# DeprecationWarning: Not passing `on_delete` to ForeignKeyField is deprecated and will be an
# error in a future release. Pass `on_delete` explicitly (e.g. on_delete=fields.CASCADE).

# Correct usage going forward:
class MyModel(Model):
    user = fields.ForeignKeyField("models.User", on_delete=fields.CASCADE)

Notes

• Backward compatible: existing code still works, just warns
• The warning fires at model class definition time (not at query time), so it's easy to catch
• Future release can make on_delete truly required by removing the sentinel fallback