preserve embedded NUL bytes in custom function text values

[dxbjavid]

callbackArgString hands a TEXT argument to a custom function by converting it with C.GoString, which stops at the first NUL byte, so a function declared with a string parameter silently loses everything after an embedded NUL in the value it was given; callbackRetText has the mirror problem and returns its string through sqlite3_result_text with a -1 length, so a returned string is cut off at its first NUL on the way back into the database. A function that filters or validates its input then only sees the bytes before the NUL, which can let the rest slip through unchecked. I read the real byte count with sqlite3_value_bytes on the way in and pass the explicit length to sqlite3_result_text on the way out, with the same oversize guard the blob path already carries since we now hand it a real length.

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Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.63%. Comparing base (18cdded) to head (b1e8d68).
⚠️ Report is 95 commits behind head on master.

Files with missing lines	Patch %	Lines
callback.go	62.50%	2 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1406      +/-   ##
==========================================
+ Coverage   47.16%   51.63%   +4.47%     
==========================================
  Files          12       13       +1     
  Lines        1533     1867     +334     
==========================================
+ Hits          723      964     +241     
- Misses        669      742      +73     
- Partials      141      161      +20     

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[mattn]

LGTM. Correctly preserves embedded NULs on both paths, guard matches the blob path, and the test covers both directions. Thanks!

Nit (non-blocking): the TEXT arg case calls sqlite3_value_bytes before sqlite3_value_text; SQLite's documented order is text-then-bytes. Consistent with the existing BLOB case and harmless on the UTF-8 path, so fine to leave.

[mattn]

Ref for the ordering note: https://www.sqlite.org/c3ref/value_blob.html

[dxbjavid]

good catch, went ahead and flipped it to text-then-bytes on the TEXT case so it follows the documented order in that ref. left the existing BLOB case as-is since it predates this and isn't really in scope here.

[mattn]

Thank you

[dxbjavid]

Glad to contribute. Thanks