Add SET DEFAULT and NO ACTION to OnDeleteType

[anxkhn]

`OnDeleteType`, the `Literal` that types the public `Field(ondelete=...)`
parameter, only listed three of the five standard SQL referential actions:
`CASCADE`, `SET NULL` and `RESTRICT`. The remaining two, `SET DEFAULT` and
`NO ACTION`, are valid everywhere else in the stack: `ondelete` is passed
straight through to SQLAlchemy's `ForeignKey(ondelete=...)`, which emits
`ON DELETE <value>` for any of them, and SQL defines exactly these five actions.

Because the literal omitted them, a type checker (mypy/pyright/ty) rejected a
perfectly valid schema, for example:

```python
class Hero(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    team_id: int | None = Field(
        default=None, foreign_key="team.id", ondelete="NO ACTION"
    )

The only workarounds were casting to Any or dropping down to sa_column.
NO ACTION is the default referential action in PostgreSQL and most databases,
and SET DEFAULT pairs naturally with a column server_default, so both are
things people legitimately want to spell out.

This adds the two missing actions to OnDeleteType. There is no runtime change
(the value was always passed through unchanged); this only corrects the static
type. Reported in #1839.

Tests

Added a parametrized regression test over the two newly typed actions that
asserts both the resulting foreign key's ondelete and the generated
ON DELETE <action> DDL, mirroring the existing
test_foreign_key_ondelete_with_annotated.

AI Disclaimer

AI used for diagnostics and drafting; the change and test were reviewed by hand.

---

## Notes for the human reviewer (Anas)

- Head branch is a placeholder name (`patch-4`) from the loop scaffold. Before you
  open the PR, consider renaming it to something conventional, e.g.
  `fix/ondelete-referential-actions`. (Branch names never reach code, only the PR.)
- No CONTRIBUTING.md, no PR template, no DCO/CLA in this repo. The commit is a
  plain Conventional Commit (`fix: ...`), no sign-off needed.
- I included an `## AI Disclaimer` section because the maintainer's own recent
  type-fix PR (#1996) uses exactly that section; keeping it is honest and matches
  house style. Drop it if you'd rather not.
- The repo's CI enforces "Coverage stays at 100%." The new test exercises the new
  literal values end to end, so coverage should hold; the maintainer's checklist
  item is `- [x] Coverage stays at 100%.` if you want to add it.