Allow SPU cache custom calls in dynamic export

[FollyCoolly]

Summary

• allow SPU cache custom calls during dynamic-shape JAX export
• add a regression test for make_cached_var/drop_cached_var in SPU dynamic compile

Tests

• uv run ruff check mplang/dialects/spu.py tests/backends/test_spu_impl.py
• uv run pytest tests/backends/test_spu_impl.py

[gemini-code-assist]

Code Review

This pull request updates the SPU dialect compilation settings to disable checks for spu.make_cached_var and spu.drop_cached_var during JAX export. It also adds a corresponding test case test_spu_dynamic_shape_allows_cache_custom_calls to verify that SPU-owned cache custom calls are allowed with dynamic shapes. There are no review comments, and I have no additional feedback to provide.

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[danqi437]

LGTM