Simplify SxPB setup parsing to requests-only format

[gendeux]

Motivation

• The SxPB test fixtures were fully pluralized to use setup.requests, so the test harness should stop supporting the legacy singular setup.request fallback.
• Removing the ad-hoc list normalization simplifies the test code and avoids divergent handling for request vs requests fields.

Description

• Updated test/server_test.py to iterate only over setup.requests via for req in cast(List[Dict[str, Any]], setup_block.get("requests", [])) and removed the singular fallback.
• Removed the normalize_list helper and its usages, and made request/response values handling use the raw values field directly.
• Updated test/server_test_cases.sxpb to replace all setup.request instances with setup.requests using the SxPB list form so fixtures and test harness match.

Testing

• Ran pytest -q and confirmed all tests passed: 8 passed.

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Codex Task