[Offload] Add support for Mipmapped Textures

[s-perron]

This commit extends the offload-test-suite infrastructure to support
textures with multiple mip levels.

• Update OutputProperties to include MipLevels.
• Update Vulkan backend to correctly create, upload, and sample from
  multi-mip images.
• Add error checking in the DirectX and Vulkan backends to validate
  texture dimensions and mip counts.
• Add docs/MipMappedTextures.md to explain the required data layout.

Comprehensive tests were added for Texture2D functions, including sampling,
gradients, and LOD calculation. Existing tests were updated to reflect
correct results under the new infrastructure.

Note: Multiple mip levels are currently restricted in the DirectX backend
via runtime error checking.

Fixes #690

Assisted-by: Gemini

[Keenuts]

LGTM, just a suggestion to improve usability

[Icohedron]

This PR broke the tests introduced by #648
The tests introduced by that PR are now failing on the CI machines with errors like

# .---command stderr------------
# | YAML:14:5: error: Buffer 'Output' size (16) does not match OutputProps dimensions (256)
# |   - Name: Output
# |     ^
# | gpu-exec: error: invalid argument
# `-----------------------------

log: https://github.com/llvm/offload-test-suite/actions/runs/22190060194/job/64174939577#step:12:439

[Keenuts]

Looks like tests added in #648 can be fixed with #758 but I see additional failures on other tests, seems like those are caused by the change of value of the Depth parameter in the pipeline output.