Fix request-metadata handling of empty HTTP probes

[tsarlandie-oai]

Summary

• reject unsupported HTTP methods before parsing a JSON-RPC body in the
  request-metadata client scenario
• return a structured JSON-RPC parse error for malformed POST bodies
• cover empty-body GET/DELETE probes and malformed POST requests with
  regression tests

This prevents a valid Streamable HTTP endpoint probe from crashing the entire
conformance runner with SyntaxError: Unexpected end of JSON input.

Fixes #408.

Validation

• npm test (440 tests passed)
• npm run typecheck
• npm run lint
• npm run build
• exercised the scenario with a real Codex HTTP client; the adapter completed
  discovery and the runner emitted checks instead of crashing