feat: add optional Pydantic / JSON schema hooks for OpenAPI body #9

Context

• openapi_add_path produces a bare 200 with no content. Users expect application/json schemas for bodies.

Options

1. Python-only: at route time, pass optional body_model=MyPydanticModel to the decorator; Rust only stores a JSON Schema string generated in Python (model_json_schema()) in RouteEntry; merge into the OpenAPI document on freeze().
2. Rust-only: stay minimal. Prefer (1) to avoid reimplementing Pydantic in Rust.

Acceptance criteria

• /openapi.json includes requestBody for at least one POST route.

Research

• Pydantic v2 JSON schema $defs; size of inline vs $ref in OAS 3.0.3.