refactor: rename BLOCK_EXPANDING/block_diffusion to BLOCK_EXTEND

[leon062112]

…k_extend

• MaskMode.BLOCK_EXPANDING -> MaskMode.BLOCK_EXTEND (enum value)
• kBlockExpanding -> kBlockExtend (C++ enum)
• BLOCK_EXPANDING -> BLOCK_EXTEND (C++ template params)
• block_expanding_prefill.cuh -> block_extend_prefill.cuh (file rename)
• q/kv_block_expanding_offset -> q/kv_block_extend_offset (members)
• block_diffusion -> block_extend (Python params/attrs/functions/files)
• _block_diffusion.py -> _block_extend.py (file rename)
• _block_diffusion_batch.py -> _block_extend_batch.py (file rename)
• test_block_diffusion_attention.py -> test_block_extend_attention.py

$ /opt/conda/bin/python3 -m pytest tests/attention/test_block_extend_attention.py -v --no-header
==================================================================================== test session starts =====================================================================================
collected 4 items                                                                                                                                                                            

tests/attention/test_block_extend_attention.py::test_block_extend_batch_generator_routing PASSED                                                                                       [ 25%]
tests/attention/test_block_extend_attention.py::test_block_extend_single_prefill_matches_reference PASSED                                                                              [ 50%]
tests/attention/test_block_extend_attention.py::test_block_extend_ragged_wrapper_matches_reference PASSED                                                                              [ 75%]
tests/attention/test_block_extend_attention.py::test_block_extend_paged_wrapper_matches_reference PASSED                                                                               [100%]

====================================================================================== warnings summary ======================================================================================