fix: add null check to untyped JSON.parse for malformed input

[cs01]

Summary

• Add runtime null check after csyyjson_parse in generateUntypedParse — malformed JSON now prints a clear error and exits instead of dereferencing null
• Before: JSON.parse("bad") on untyped path → null pointer dereference → segfault
• After: Error: JSON.parse() failed — invalid JSON input + exit code 1
• Note: this code path is currently unused (typed parse required by error diagnostic), but guards against future use
• Completes yyjson null-check audit: all csyyjson_parse call sites now check for null

Test plan

• npm run verify:quick passes (tests + Stage 1 self-hosting)

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