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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