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)

src/file_tree.c

@@ -411,7 +411,7 @@ int file_tree_write(FileTree *ft, string path,
 
 int file_tree_read(FileTree *ft, string path,
     uint64_t off, uint64_t len, uint64_t *chunk_size,
-    SHA256 *hashes, int max_hashes)
+    SHA256 *hashes, int max_hashes, uint64_t *actual_bytes)
 {
     int num_comps;
     string comps[MAX_COMPS];
@@ -432,6 +432,18 @@ int file_tree_read(FileTree *ft, string path,
 
     *chunk_size = f->chunk_size;
 
+    // Calculate file size (number of chunks * chunk size)
+    uint64_t file_size = f->num_chunks * f->chunk_size;
+
+    // Calculate actual bytes that can be read
+    if (off >= file_size) {
+        *actual_bytes = 0;
+    } else if (off + len > file_size) {
+        *actual_bytes = file_size - off;
+    } else {
+        *actual_bytes = len;
+    }
+
     if (len == 0)
         return 0;