Track actual bytes read in read operations

Add functionality to determine the actual number of bytes read during
read operations, making it possible to detect when a read was truncated
because it went past the end of the file.

Changes:
- Modified file_tree_read() to calculate and return actual_bytes via
  new output parameter
- Updated metadata server to send actual_bytes in READ_SUCCESS messages
- Added bytes_read field to ToastyResult structure
- Modified client to parse, store, and report actual bytes read
- Updated toasty_read() to return the actual number of bytes read
  instead of always returning 0
- Fixed web server to use bytes_read field instead of non-existent
  count field

This allows clients to distinguish between:
- Reading zeros because the file is sparse (has holes)
- Reading past the end of the file (truncated read)

inc/ToastyFS.h

@@ -159,6 +159,7 @@ typedef struct {
     ToastyResultType type;
     ToastyListing    listing;
     void *user;
+    int bytes_read; // For read operations: actual number of bytes read
 } ToastyResult;
 
 // If the operation specified by "handle" is complete,