[NFC][SYCL] Add sycl/detail/fwd/accessor.hpp helper

[aelovikov-intel]

Similar to #20049

[Copilot]

Pull Request Overview

Adds a new forward declaration header sycl/detail/fwd/accessor.hpp to centralize accessor-related forward declarations and eliminates redundant forward declarations throughout the codebase.

• Consolidates all accessor forward declarations into a single header file
• Removes duplicate forward declarations from multiple files
• Updates include dependency tests to reflect the new include structure

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sycl/include/sycl/detail/fwd/accessor.hpp	Creates new header with consolidated accessor forward declarations
sycl/include/sycl/accessor.hpp	Removes redundant forward declaration and adds default template parameters
sycl/include/sycl/buffer.hpp	Removes local forward declaration, includes new header
sycl/include/sycl/handler.hpp	Removes redundant image_accessor forward declaration
sycl/include/sycl/image.hpp	Removes multiple accessor forward declarations
sycl/include/sycl/multi_ptr.hpp	Removes local accessor forward declarations
sycl/include/sycl/sampler.hpp	Removes image_accessor forward declaration
sycl/include/sycl/properties/accessor_properties.hpp	Removes extensive accessor forward declarations
sycl/include/sycl/ext/oneapi/accessor_property_list.hpp	Removes accessor forward declaration
sycl/include/sycl/detail/accessor_iterator.hpp	Removes accessor forward declaration
sycl/test/include_deps/sycl_*.cpp	Updates test expectations for new include order