feat(validation): runtime body_model validation with 422 errors

[ZhuchkaTriplesix]

Context

• Route decorators accept body_model (Pydantic v2) and body_schema (dict) for OpenAPI (oxyroute/app.py, src/lib.rs).
• JSON body is parsed in src/dispatch.rs but not validated against the model/schema at runtime.

Problem

OpenAPI documents request shapes that the framework does not enforce. Handlers must manually validate Pydantic models, unlike FastAPI-style DX.

Proposed change

• Store validation metadata on RouteEntry at registration.
• Before the handler runs, validate parsed JSON against body_model (Pydantic) or optional JSON Schema.
• Return 422 with a normalized error shape on validation failure.
• Decide handler API: validated model instance vs existing json dict (document breaking vs additive choice).

Files

• oxyroute/app.py, src/lib.rs, src/dispatch.rs, src/state.rs
• docs/openapi.md, docs/handlers.md, tests (Pydantic v2)

Acceptance criteria

• Invalid payload returns 422 without calling the route handler.
• Valid payload reaches handler with documented shape.
• OpenAPI generation unchanged or improved.
• Tests cover Pydantic validation errors.

Framework impact

Less boilerplate in handlers; closer to FastAPI request validation.