fix: alloc free off by one (#8886)

Fix provided by @jcollie 

The swift `open_config` action was triggering an allocation error
`error(gpa): Allocation size 41 bytes does not match free size 40.`.

> A string that was created as a `[:0]const u8` was cast to `[]const u8`
and then freed. The sentinel is the off-by-one.

@jcollie 

For full context, see
https://discord.com/channels/1005603569187160125/1420367156071239820

Co-authored-by: Jeffrey C. Ollie <jcollie@dmacc.edu>

include/ghostty.h

@@ -353,6 +353,7 @@ typedef struct {
 typedef struct {
   const char* ptr;
   uintptr_t len;
+  bool sentinel;
 } ghostty_string_s;
 
 typedef struct {