Fall back to schema-root example(s) for body examples

#30 surfaces media-type-level `examples`/`example` in op docs. Some specs (e.g. Anthropic's `POST /v1/messages`) instead put a complete whole-payload example *inside* the root request schema (`schema.example` in 3.0, `schema.examples` array in 3.1) — currently dropped.

Follow-up: in `_body_params`, when the media type has no examples, resolve the schema and read its root `example` (or first of `examples`) as a `{"default": {"value": ...}}` body example. Root-level only — property-level examples are a separate issue.

Coverage scan: Anthropic 5/13 body ops have only this form (incl. `/v1/messages`); GitHub 10; Cloudflare 25.

Start at `_body_params` and inspect how media-type-level `examples`/`example` are handled and how the request schema is resolved. Done means that, when no media-type examples exist, a root `schema.example` or first `schema.examples` value becomes the default body example, while media-type examples take precedence and property-level examples remain out of scope.

Written by the indexing model from the issue text.