fix(responses): avoid parsing incomplete structured output

[cyphercodes]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

Fixes #1839.

• Skip auto-parsing structured Responses output when the response is not completed, so incomplete responses can be inspected via status / incomplete_details instead of surfacing a JSON parse error first.
• Keep completed response parsing behavior unchanged and add focused coverage for both paths.

Additional context & links

Verification:

• yarn test tests/lib/ResponsesParser.test.ts --runInBand
• yarn build
• ./node_modules/.bin/prettier --check src/lib/ResponsesParser.ts tests/lib/ResponsesParser.test.ts
• ./node_modules/.bin/eslint src/lib/ResponsesParser.ts tests/lib/ResponsesParser.test.ts
• git diff --check