feat(core)!: request body is bare JSON; response body key is 'schema'

[rizukirr]

Summary

Breaking contract-format change. Bodies are written more directly:

• Request body is now the raw JSON payload written directly under
  request, with no example wrapper.
• Response body moves from example to the response's schema key.

// before
"request":  { "example": { "name": "Rizki Rakasiwi" } },
"responses": [ { "code": 200, "description": "OK", "example": { "status": 200 } } ]

// after
"request":  { "name": "Rizki Rakasiwi" },
"responses": [ { "code": 200, "description": "OK", "schema": { "status": 200 } } ]

Changes

• apic-core: JsonContent.request is now Option<Value> (the RequestBody
  wrapper struct is removed); Response.example renamed to Response.schema.
  Threaded through the edit model (from_contract / to_json), the Postman
  converter, and their tests.
• CLI/TUI: the apic read renderer and the TUI editor read the new shape; the
  new-contract seed logic clears the new body keys (request / schema).
• Examples: all example/ contracts migrated to the new shape.
• Docs: README "Contract format" and CHANGELOG updated.

Old { "example": ... } body contracts do not carry over and must be updated
(there is no back-compat shim).

Notes

• Documented under CHANGELOG [Unreleased]; the version is left at 0.4.0, bump
  to 0.5.0 (breaking) when you cut the next release.
• v0.4.0 (already released) is untouched.

Tests

cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings are clean; cargo test --workspace passes (257 tests). Every migrated
example/ contract parses under the new format.