[TensorRT] Deduplicate context path helpers (#29640)
## Summary
- move duplicated TensorRT context path helpers into
`core/common/path_utils.h`
- reuse the shared helpers from the TensorRT and NvTensorRT RTX
providers
- add cross-platform unit coverage for path construction and validation
behavior
## Testing
- `lintrunner`
- `onnxruntime_test_all.exe
--gtest_filter=PathUtilsTest.*:PathTest.*:PathValidationTest.*`
- `onnxruntime_test_all.exe` (1,843 tests: 1,816 passed, 27 skipped)
Co-authored-by: Gopalakrishnan Nallasamy <gopalakrishnan.nallasamy@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
