[GPU] Fix reshape runtime padding propagation when cropped axis is dropped (#35616)
### Description of the issue(symptom, root-cause, how it was resolved)
- **Symptom**: GPU produced wrong box coordinates / accuracy mismatch vs
CPU on a detection model with a `VariadicSplit (last axis, lengths
[1,1,1,1])` followed by base-mode `Reshape`s that drop the cropped
size-1 axis (`[N,M,1] -> [N,M]`).
- **Root cause**: In `is_runtime_propagatable_padding()` (last-axis crop
branch), the trailing-dim matching loop exits immediately when the
cropped axis is size 1, returning true even when the reshape removes
that axis. With no output dim left to carry the crop's dynamic padding,
the four sibling reshape outputs propagated zero padding and ended up
aliased to the same base-buffer offset, so downstream consumers read
interleaved channel data as if it were contiguous.
- **Fix**: Track the number of trailing output dims actually matched
against the cropped axis (`matched_trailing_dims`) and reject
propagation when none were matched and the reshape reduces rank — i.e.
the cropped axis is genuinely dropped. Identity reshapes that preserve
it (`[N,M,1] -> [N,M,1]`) remain eligible.
#### The code and line that caused this issue (if it is not changed
directly)
- `src/plugins/intel_gpu/src/graph/include/reshape_inst.h`
#### Reproduction step and snapshot (if applicable. Do not attach for
customer model)
- ./ov_gpu_unit_tests
--gtest_filter='prepare_buffer_fusing.in_place_crop_dynamic_last_axis_split_to_collapsing_reshape'
#### Problematic graph
<img width="670" height="555" alt="image"
src="https://github.com/user-attachments/assets/10790a77-7887-408d-9133-0d883c9d4ee1"
/>
#### Checklist
- [x] Is it a proper fix? (not a workaround)
- [x] Did you include test case for this fix, if necessary?
- [x] Did you review existing test that can be extended to cover this
scenario? Which test did you review?
- Reviewed existing tests under `prepare_buffer_fusing` in
`src/plugins/intel_gpu/tests/unit/passes/prepare_buffer_fusing_test.cpp`
(in-place crop / reshape padding tests)
### Tickets:
- [CVS-185921](https://jira.devtools.intel.com/browse/CVS-185921)
Co-authored-by: Copilot <copilot@github.com>