test: cover parse case edge cases

[laxmanacharya8]

Problem

The case parser was only exercised through successful command-line parses of the two example vectors. Parser rejection behavior, explicit input formats, and v2 serialization stability did not have focused regression coverage.

Root Cause

make check-scripts invoked parse_case.py for valid fixtures but did not run assertions over malformed inputs or parser helper behavior.

Change Summary

• add a focused parse_case_smoke.py suite
• cover valid v1/v2 fixtures and a lossless v2 serialize/parse round trip
• cover empty input, unknown versions, trailing bytes, unsupported flags, and inconsistent recipient flags
• cover automatic hex decoding, forced binary reads, and invalid hex rejection
• run the smoke suite from make check-scripts and document it

Testing

• python3 scripts/parse_case_smoke.py — passed
• python3 -m py_compile scripts/parse_case_smoke.py — passed
• make check-claims check-comments PYTHON=python3 — passed
• git diff --check — passed
• make check-scripts — new parser smoke passed; the existing C++ worker smoke could not compile locally because secp256k1.h is not installed

Primary Sources

• Existing v1 and v2 fixtures under tests/vectors/
• Existing parser contract in scripts/parse_case.py

Release Impact

Test-only change; no runtime or release behavior changes.

Checklist

• I reproduced the issue or explained the deterministic correctness argument.
• I added or updated tests where appropriate.
• I listed the exact commands I ran.
• I kept changes deterministic.
• I updated documentation when behavior changed.
• I did not describe outcomes beyond the evidence in this patch.

[shuv-amp]

ACK

Coverage looks right.