feat(parameters): describe Parameters TypedDicts and Constraint base

[Kamilbenkirane]

Summary

Annotate every field in the modality Parameters TypedDicts with Annotated[T, Field(description=...)] and add an optional description: str | None = None slot to the Constraint base.

Together, these give downstream consumers (playgrounds, LLM-tool wrappers, OpenAPI generators) access to per-parameter human-readable metadata via the standard Pydantic introspection path (get_type_hints(ImageParameters, include_extras=True) + model.parameter_constraints[name].description).

Why

Today, ImageParameters / AudioParameters / VideoParameters / TextParameters / EmbeddingsParameters carry bare types with no per-field documentation. Constraints are pure validators with no description slot. Consumers hand-label in their own UI/schema code; there's no canonical way to ask "what does this parameter mean?" at the celeste level.

Design

• Descriptions state semantic meaning, not valid values. Enumerating values (like '16:9' / '1:1' for aspect ratio) would hardcode one provider's choices into every consumer's schema. Instead, each per-model parameter_constraints entry (Choice, Range, Dimensions, etc.) continues to own valid-value enumeration; the TypedDict annotation tells you what the field means.
• Constraint-level description is a per-model override. When a model wants to narrow or restate the generic semantic (e.g. "this model's aspect_ratio only accepts 16:9 or 1:1"), it sets description= on the constraint instance in its parameter_constraints={...} dict.
• No new module, no new helper, no new behavior. Primitives only.

Test plan

• uv run pytest tests/unit_tests — 594 tests pass.
• uv run ruff check src/celeste tests/unit_tests/test_parameters.py — clean.
• uv run mypy src/celeste/modalities/*/parameters.py src/celeste/constraints.py — clean.
• New tests assert every modality's TypedDict has a Field description on every field, base types survive under Annotated, and Constraint.description defaults to None and flows to subclasses.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.