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>

tests/src/unit-cbor.cpp

@@ -2785,10 +2785,10 @@ TEST_CASE("CBOR large strings and binaries (chunked reader)")
     // sure roundtripping is correct for lengths around and beyond the internal
     // chunk size (4096 bytes), for both vector (iterator) and pointer inputs.
     for (const std::size_t len :
-    {
-        std::size_t{0}, std::size_t{1}, std::size_t{4095}, std::size_t{4096},
-        std::size_t{4097}, std::size_t{8192}, std::size_t{100000}
-    })
+            {
+                std::size_t{0}, std::size_t{1}, std::size_t{4095}, std::size_t{4096},
+                std::size_t{4097}, std::size_t{8192}, std::size_t{100000}
+            })
     {
         CAPTURE(len);