bound .mat array name length in halide_debug_to_file

[Nashit-h]

The .mat writer in halide_debug_to_file builds an array name from the filename's base name (the text between the last slash and the extension) and copies it into a fixed char array_name[256] with no length check, so a filename whose base name is longer than 256 bytes overflows the stack buffer. Reject an oversized name before the copy, using the same halide_error_code_debug_to_file_failed return the writer already uses for oversized headers and payloads.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.90%. Comparing base (abbf7ff) to head (a657e68).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9379      +/-   ##
==========================================
- Coverage   70.14%   69.90%   -0.24%     
==========================================
  Files         260      260              
  Lines       79287    79287              
  Branches    19327    19327              
==========================================
- Hits        55617    55429     -188     
- Misses      17899    17944      +45     
- Partials     5771     5914     +143     

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