fix: ensure direction is char array before passing to binary_search MEX

Octave can pass string objects instead of char arrays in certain
contexts, causing the MEX input validation to reject them. Wrapping
with char() ensures compatibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: HanSur94

libs/FastPlot/binary_search.m

@@ -48,7 +48,7 @@
 
     % Delegate to MEX for speed when available
     if useMex
-        idx = binary_search_mex(x, val, direction);
+        idx = binary_search_mex(x, val, char(direction));
         return;
     end
 

libs/FastPlot/private/binary_search.m

@@ -44,7 +44,7 @@
 
     % Fast path: MEX implementation
     if useMex
-        idx = binary_search_mex(x, val, direction);
+        idx = binary_search_mex(x, val, char(direction));
         return;
     end