Add Unicode replacement and surrogate tolerance flags

[edsiper]

Add YYJSON_READ_REPLACE_INVALID_UNICODE and YYJSON_READ_ALLOW_INVALID_SURROGATE flags to enable graceful handling of malformed Unicode in JSON strings.

YYJSON_READ_REPLACE_INVALID_UNICODE replaces all invalid Unicode sequences with U+FFFD replacement character, ensuring output is always valid UTF-8. Handles invalid \uXXXX escapes, unpaired surrogates, invalid UTF-8 bytes, and control characters.

YYJSON_READ_ALLOW_INVALID_SURROGATE permits unpaired surrogate code units in \uXXXX escapes, encoding them as UTF-8.

Key changes

• Modified read_uni_esc() to accept flags parameter
• Added replacement logic throughout string parsing paths
• Replacement flag implicitly enables unicode tolerance flags
• Added comprehensive test coverage
• Flags not supported in incremental parsing mode

why ?

• This enables robust parsing of real-world JSON with encoding errors while maintaining strict standards compliance by default.
• While we would love to live in strict parsing mode, we have to deal with malformed data, dealing with invalid bytes is times expensive in the caller than handling this directly inside yyjson

[codecov]

Codecov Report

❌ Patch coverage is 70.93596% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.79%. Comparing base (706ae61) to head (825aa37).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/yyjson.c	70.79%	59 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
- Coverage   98.50%   97.79%   -0.72%     
==========================================
  Files           2        2              
  Lines        9040     9255     +215     
==========================================
+ Hits         8905     9051     +146     
- Misses        135      204      +69     

Flag	Coverage Δ
unittests	97.79% <70.93%> (-0.72%)	⬇️

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[ibireme]

Thanks for the PR!

Replacing invalid chars with U+FFFD makes sense — I already support do that on the serialization side.

The main issue is during parsing: the unescaping process is performed in-place, so the unescaped character must not be longer than the original escape sequence. Since U+FFFD takes 3 bytes in UTF-8, it cannot safely replace invalid sequences that are only 1–2 bytes.

For example: "abcdefg", `d` is escaped
    src         
     │          
┌────▼─────────┐
│"abd\u0064efg"│
└────▲─────────┘
     │          
    dst         
          src   
           │    
┌──────────▼───┐
│"abcdu0064efg"│
└─────▲────────┘
      │         
     dst        
             src
              │ 
┌─────────────▼┐
│"abcdefg 4efg"│
└────────▲─────┘
         │      
        dst     

I'd suggest introduce a new string subtype (e.g. YYJSON_SUBTYPE_UNIERR) to indicate strings containing invalid Unicode. This would be similar to the existing YYJSON_SUBTYPE_NOESC, but it would still require the caller to detect and handle such cases.

[edsiper]

@ibireme thanks for your feedback. I have updated the patch with the suggestions (majority of the changes were done with Codex)

[ibireme]

I thought a bit more about this and I think we should split bad strings into three categories:

1. Invalid UTF-8 bytes
   Since yyjson decodes in-place, we can't always replace them with U+FFFD. So REPLACE_INVALID_UNICODE doesn't really help here.

2. Invalid single-character escapes
   Right now we just error on things like \Q.
   For reference, JSON5 just drops the \ and outputs Q.

3. Invalid \uXXXX sequences
   These are trickier since the error could come from many places.
   The current PR sometimes gives surprising results, e.g. \u😀 -> \uŸ˜€, \uA_CD -> \uACD.

The original goal was to turn malformed strings into valid UTF-8 internally. But with the current in-place parsing design, callers still need to check for invalid strings using SUBTYPE_UNIERR.

So my suggestion is keep using the existing READ_ALLOW_INVALID_UNICODE, just mark invalid strings with SUBTYPE_UNIERR.
Add a new flag READ_ALLOW_INVALID_ESCAPE to handle all bad escapes (either keep them as-is or just drop the \).

What do you think?