Remove dpctl_ext.tensor C-API (#2830)

This PR removes the unused external C-API from `dpctl_ext.tensor` and
replaces function pointer calls with direct struct member access.

Changes:
1. Remove all `cdef api` functions from `_usmarray.pyx`
2. Delete `dpctl_ext_capi.h` and `DpctlExtCAPI` CMake interface library
3. Update `dpnp4pybind11.hpp` to access `PyUSMArrayObject` members
directly
4. Update build configuration

Author: vlad-perevezentsev

CMakeLists.txt

@@ -344,16 +344,5 @@ if(DEFINED SKBUILD)
     set(_ignore_me ${SKBUILD})
 endif()
 
-# DpctlExtCAPI: Interface library for dpctl_ext C-API
-# Provides access to:
-# 1. Public C-API headers from dpctl_ext/apis/include
-# 2. Generated Cython headers via per-target header interface libraries
-
-add_library(DpctlExtCAPI INTERFACE)
-target_include_directories(
-    DpctlExtCAPI
-    INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/dpctl_ext/apis/include
-)
-
 add_subdirectory(dpctl_ext)
 add_subdirectory(dpnp)

dpctl_ext/CMakeLists.txt

@@ -199,12 +199,4 @@ function(build_dpctl_ext _trgt _src _dest)
     target_include_directories(${_trgt_headers} INTERFACE ${_trgt_headers_dir})
 endfunction()
 
-# Install dpctl_ext C-API headers (similar to dpctl's C-API installation)
-install(
-    DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/apis/include/
-    DESTINATION ${CMAKE_INSTALL_PREFIX}/dpctl_ext/include
-    FILES_MATCHING
-    REGEX "\\.h(pp)?$"
-)
-
 add_subdirectory(tensor)

dpctl_ext/apis/include/dpctl_ext_capi.h

@@ -34,8 +34,6 @@ foreach(_cy_file ${_cython_sources})
     get_filename_component(_trgt ${_cy_file} NAME_WLE)
     build_dpctl_ext(${_trgt} ${_cy_file} "dpctl_ext/tensor" RELATIVE_PATH "..")
     target_include_directories(${_trgt} PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include)
-    # target_link_libraries(DpctlCAPI INTERFACE ${_trgt}_headers)
-    target_link_libraries(DpctlExtCAPI INTERFACE ${_trgt}_headers)
 endforeach()
 
 if(WIN32)
@@ -318,6 +316,7 @@ foreach(python_module_name ${_py_trgts})
             ${Dpctl_INCLUDE_DIR}
             ${CMAKE_CURRENT_SOURCE_DIR}/libtensor/include
             ${CMAKE_CURRENT_SOURCE_DIR}/libtensor/source/
+            ${CMAKE_BINARY_DIR} # For generated Cython headers
     )
     target_link_options(${python_module_name} PRIVATE ${_linker_options})
     if(DPCTL_GENERATE_COVERAGE)
@@ -343,11 +342,8 @@ foreach(python_module_name ${_py_trgts})
             PRIVATE ${_dpnp_sycl_target_link_options}
         )
     endif()
-    # TODO: update source so they reference individual libraries instead of
-    #   dpctl4pybind11.hpp. It will allow to simplify dependency tree
-    # NOTE: dpctl C-API is resolved at runtime via Python
-    # target_link_libraries(${python_module_name} PRIVATE DpctlCAPI)
-    target_link_libraries(${python_module_name} PRIVATE DpctlExtCAPI)
+    # Ensure Cython modules build first so _usmarray.h exists
+    add_dependencies(${python_module_name} _usmarray)
     if(DPNP_WITH_REDIST)
         set_target_properties(
             ${python_module_name}