fix(openapi): send empty required JSON request bodies - #442

A generated OpenAPI tool accepts {} when a required JSON object's properties are all optional, but currently sends no HTTP body. Send {} in that case, including closed empty objects and required JSON bodies without a schema. This also works when only query arguments are supplied.

Omitted optional bodies, form bodies, and explicit whole-body inputs retain their existing behavior. This follows the dictionary-body investigation in #381 and fixes the separate required flattened-body presence case.

Validation:

• The two focused suites show 10 expected assertion failures on the unchanged base and 159 passes with the fix. The 24 added cases include public MCP calls for OpenAPI 3.0 and 3.1 and omission/encoding controls.
• Windows / Node 22.20.0 / pnpm 10.32.1: repository-wide Prettier, ESLint, TypeScript, 1,055 offline tests and the normal package build passed. The DiskStore suite was executed separately: 14 passed, 2 POSIX permission tests skipped, and the same 2 Windows rename failures reproduced on both base and head.
• All 15 OpenAPI network benchmark/live tests passed, including the live Petstore calls.

The existing Deno 1.41.3 JSR dry-run still fails on both base and head with the same 10 dependency declaration diagnostics; their outputs are byte-identical. Linux / Node 24 remains for upstream CI.

AI assistance was used for investigation, implementation, and tests.