[GPU] GQA i8/i4: address review - kernel unit tests + readable static cases

Review feedback on the quantized-KV GroupQueryAttention tests:

- Add GPU kernel unit tests for the newly enabled i8/u8 data types (ref:
  scatter_update_gpu_test.cpp / slice_gpu_test.cpp):
  * scatter_update_gpu_{i8,u8}.d8111_axisB - templated int scatter (byte movement).
  * slice_gpu_i8.bfyx - dedicated small-value i8 case (the shared TYPED_TEST data
    exceeds the i8 range, so a separate case rather than extending DataTypes).
- Rewrite the *_static GQA test cases to declare named query/past_key/past_value/
  expected_* variables with tensor-shape comments (matching the dynamic cases'
  style) instead of positional literals passed to the helper, and note that the
  static cases complement the existing dynamic (Slice+Concat) onnx_model_gqa_*
  cases.

All green: scatter i8/u8 + slice i8 unit tests pass; GQA quant suite 27/27
(i8/i4 x per_channel/per_tensor x dynamic/static across IE_GPU/IE_CPU/INTERPRETER).

Author: ankitm3k