fix(ipa): handle delete request bodies in IPA-117

[andmatei]

Fix IPA-117 DELETE request body crash

Steps to reproduce:

• Add a DELETE operation with requestBody.content[*].
• Run Spectral with the IPA ruleset.
• Before this change, IPA-117 threw: Cannot read properties of null (reading startsWith).

Failing fixture before:

paths:
  /resource/{id}:
    delete:
      requestBody:
        content:
          application/vnd.atlas.2024-08-05+json:
            schema:
              type: object
      responses:
        "204":
          content:
            application/vnd.atlas.2024-08-05+json: {}

Why it failed:

• IPA-117 evaluates both request body content and response content.
• Request body paths do not have a response code, so httpResponseCode was null.
• The DELETE response exemption called httpResponseCode.startsWith("2") anyway.

After this change:

• The same fixture no longer produces an internal error.
• A DELETE request body with a schema passes IPA-117.
• A DELETE request body without a schema is still reported by IPA-117.

What changed:

• Guard the DELETE 2xx response-code check with optional chaining.
• Add regression tests for valid and invalid DELETE request bodies.