Move dpctl_ext.tensor to dpnp.tensor

[vlad-perevezentsev]

This PR proposes a refactoring that migrates dpctl_ext.tensor module into dpnp package as dpnp.tensor

Changes:

1. Moved dpctl_ext/tensor/ directory to dpnp/tensor/
2. Updated all imports from dpctl_ext.tensor to dpnp.tensor across the codebase
3. Consolidated build: removed dpctl_ext/CMakeLists.txt, added build_dpnp_tensor_ext() to dpnp/CMakeLists.txt
4. Added DPNP_BUILD_COMPONENTS CMake option (ALL/TENSOR_ONLY/SKIP_TENSOR) for staged builds
5. Split coverage workflow into two steps to avoid memory issues
6. Updated include paths in all backend extension CMake files
7. Removed dpctl_ext/ directory and cleaned up .gitignore

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• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2829/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev3=py313h509198e_336 ran successfully.
Passed: 1355
Failed: 2
Skipped: 16