Fix GCC narrowing warning and address review feedback

- get_bytes(): replace `len -= static_cast<NumberType>(wanted)` with an
  explicit `len = static_cast<NumberType>(len - ...)`. The compound
  assignment promoted the operands to int and narrowed back, which GCC
  rejects under -Werror=arith-conversion when NumberType is narrower than
  int (e.g. UBJSON's int16 length). This also fixes the CodeQL build.
- get_bytes(): add JSON_ASSERT(read == wanted) documenting that
  get_elements() never returns more than requested (per review).
- iterator_input_adapter: de-duplicate the iterator_is_contiguous
  definition with the #if inside the initializer (per review).
- Add a comment explaining why the memcpy source is &*current (needed for
  non-pointer contiguous iterators), and drop the redundant parentheses.
- Run astyle on the new unit test so the amalgamation/format check passes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

include/nlohmann/detail/input/binary_reader.hpp

@@ -2975,13 +2975,16 @@ class binary_reader
             if (JSON_HEDLEY_UNLIKELY(read < wanted))
             {
                 // premature end of input: shrink to what was actually read and
-                // report the failure at the first missing byte
+                // report the failure at the first missing byte (same position
+                // accounting as get_to() for partial number reads)
                 result.resize(old_size + read);
                 ++chars_read;
                 current = char_traits<char_type>::eof();
                 return unexpect_eof(format, context);
             }
-            len -= static_cast<NumberType>(wanted);
+            // a full chunk was read; get_elements() never returns more than requested
+            JSON_ASSERT(read == wanted);
+            len = static_cast<NumberType>(len - static_cast<NumberType>(wanted));
         }
         return true;
     }