[allmonday]

Fixes #320

What changed

1. compose SDL render (defect 1) — _render_struct_type_sdl decided the ! suffix with _is_optional alone, which cannot see a None member inside a Literal (verified: _is_optional(Literal["draft", None]) is False — the Literal branch in _is_optional compares type(None) against the member values, so it never fires). Its sibling _build_type_ref in the same module already handled this via literal_info. Both renderers now agree:

type LiteralTaskDTO {
  """Allowed values: draft (or null)"""
  note: String        # was String! — introspection already rendered SCALAR String
}

2. entity output SDL (defect 2, option A) — map_to_sdl no longer forces NON_NULL on output for nullable annotations (Optional[T] / T | None / Literal[..., None]). Output SDL now matches map_to_introspection and the Python annotation itself:

type TaskLiteEntity {
  ...
  """Allowed values: draft (or null)"""
  note: String        # was String!
  memo: String        # Optional[str] output — was String! by convention
}

⚠️ Behavioral change: every nullable output field on the entity-first SDL loses its ! suffix. Introspection output was already nullable for all of these (entity introspection never emitted NON_NULL), so this aligns the two renderers rather than changing what clients execute against.

3. Cleanup (from the issue) —

• New annotation_is_nullable(annotation) in graphql/type_mapping.py is the single predicate for NON_NULL decisions; all former not _is_optional(...) force_non_null computations in compose_schema.py now use it so SDL and type-ref renders cannot drift again.
• _optional_core dedups the Optional-unwrap step previously copied in literal_info and _literal_constraint.
• literal_allowed_values removed — no library callers since fix: preserve nullability in Literal descriptions #318 (tests updated; its coverage is subsumed by the fix: preserve nullability in Literal descriptions #318 parametrized description tests).

Verification

• pytest tests/: 1005 passed, 1 skipped; pytest demo/ (CI also runs it): 92 passed.
• ruff check: passed.
• End-to-end: regenerated every scenario in the Output-side nullable Literal fields render NON_NULL in SDL while introspection and descriptions say nullable #320 matrix — entity SDL output, compose method_sdl, compose introspection (unchanged, was already correct), input side (unchanged) — all consistent; non-nullable literals (status: String!, enabled: Boolean!) keep their !.

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