test: add error message quality validation suite

[membphis]

Background

qjson reports parse errors with byte offsets (qjson_format_error), but there's no systematic validation that error messages are:

• Consistent across similar error types
• Informative enough for debugging
• Stable across versions (no accidental message changes)

V8 maintains error message consistency as part of its test suite.

Goal

Add a test suite that validates error message quality and stability.

Scope

Validation Dimensions

1. Offset accuracy: Error offset points to the actual problematic byte
2. Message clarity: Message describes what was expected vs. found
3. Consistency: Similar errors produce similar message patterns
4. Completeness: All error codes have corresponding message templates
5. Stability: Message format doesn't change unintentionally

Test Cases

Input	Expected Message Pattern
{	parse error at byte 1: unexpected end of input, expected ...
{"a":}	parse error at byte 5: unexpected '}', expected value
[01]	invalid number at byte 1: leading zeros not allowed
{"\xff"}	invalid UTF-8 at byte 2: ...
[[[[...1024 deep	max depth exceeded at byte N

Snapshot Testing

Consider snapshot-based testing: store expected messages in files, fail on drift.

Acceptance Criteria

• Test file covering all qjson_err variants with expected message patterns
• Each error code has at least one test case validating its message
• Messages are validated for presence of: error type, byte offset, context hint
• CI fails if message format changes unexpectedly (snapshot or pattern match)

Technical Notes

• Build on existing tests/ffi_parse_error_offsets.rs
• Use qjson_format_error FFI function for message generation
• Consider adding line:column in addition to byte offset (future enhancement)

References

• Current error formatting: src/error.rs, qjson_format_error in src/ffi.rs
• Existing offset tests: tests/ffi_parse_error_offsets.rs

[coderabbitai]

🔗 Related PRs

#43 - test: add cjson and simdjson fixture coverage [merged]
#51 - perf: skip redundant validation in eager decode, inline depth check [merged]
#78 - test(fuzz): add nesting-depth stack-safety target [merged]
#141 - test: add FFI grammar corpus [merged]
#144 - test: strengthen lazy phase2 fuzz replay [merged]

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