fix(extract): snap docstring truncation to UTF-8 codepoint boundary (#1017)

[harshitaajoshi]

Fixes #1017

What does this PR do?

Docstring/comment truncation at MAX_COMMENT_LEN (500 bytes) in extract_comment_text() cuts at a raw byte offset, which can split a multi-byte UTF-8 character and leave an incomplete byte sequence in the stored nodes.properties JSON. This breaks any strict UTF-8 consumer reading the SQLite database (e.g. Python's sqlite3 module).

This PR snaps the truncation point back to the last complete UTF-8 codepoint boundary in extract_comment_text(), and adds the same guard to the append_json_string() escape loops in both pass_definitions.c and pass_parallel.c (kept in sync per codebase convention) as defense in depth.

A regression test (docstring_utf8_truncation_boundary_issue1017) crafts a Go comment with 3-byte CJK characters straddling the 500-byte boundary and validates every byte of the extracted docstring is well-formed UTF-8.

Checklist

• Every commit is signed off (git commit -s) -- required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally ( make -f Makefile.cbm test )
  ASan hangs on macOS Tahoe 26.5 (Apple Clang 17) for any binary, including a trivial int main(){return 0;}. All 6294 tests pass when built without sanitizers (SANITIZE="").
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)