request-metadata client scenario crashes on empty-body GET

[tsarlandie-oai]

Bug

The request-metadata client scenario assumes every request has a JSON body.
Its HTTP handler unconditionally calls JSON.parse(body), so a client that
sends a GET request to the endpoint causes the conformance runner to crash:

SyntaxError: Unexpected end of JSON input
    at JSON.parse
    at IncomingMessage.<anonymous>

Streamable HTTP clients may send GET requests to open an SSE stream or probe
the endpoint. A conformance fixture should respond to those requests without
terminating the entire runner.

Reproduction

node dist/index.js client \
  --command '<client that sends GET before/alongside POST>' \
  --scenario request-metadata \
  --spec-version 2026-07-28

The runner exits before writing checks.json.

Proposed fix

Because this scenario only evaluates metadata on POST requests:

• Reject unsupported non-POST methods with 405 before reading a request body.
• Only parse JSON for POST requests.
• Return a JSON-RPC parse error for malformed POST bodies instead of crashing.

Add regression tests showing that empty-body GET and DELETE requests do not
crash the scenario and that malformed POST input receives a structured error.