[Bugfix] Fix hybrid model tests

[DarkLight1337]

Fix failing tests in test_hybrid.py. Also, consolidate the tests from test_mamba.py into this file to avoid duplicated code.

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[tlrmchlsmth]

Thank you for doing this - LGTM when green

Some comments on the test failures

1. Looks like some tests aren't getting cleaned up properly

2. This is a bug -- the ConstantSizeCache unfortunately needs to look at the attention metadata and is choking on xgrammar being there instead of flash attn

[2025-04-25T12:45:10Z] FAILED models/decoder_only/language/test_hybrid.py::test_multistep_correctness[64-float-ai21labs/Jamba-tiny-dev] - ValueError: Multi-Step not supported for attention backend: XFORMERS. Set VLLM_ATTENTION_BACKEND to a value from ['FLASH_ATTN', 'ROCM_FLASH', 'FLASHINFER', 'NO_ATTENTION'].

3. I've found the hybrid and mamba tests to be pretty flakey... Somewhat suspect some numerical instability either in the models or the implementation but I don't have any hints as to what would be going wrong

[2025-04-25T12:45:10Z] FAILED models/decoder_only/language/test_hybrid.py::test_mamba_prefill_chunking[7-bfloat16-Zyphra/Zamba2-1.2B-instruct] - AssertionError: Test1:
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  | [2025-04-25T12:45:10Z] chunked:	'Briefly describe the major milestones in the development of artificial intelligence from 1950 to 2020.\n\nC. Discuss the impact'
  | [2025-04-25T12:45:10Z] non_chunked:	'Briefly describe the major milestones in the development of artificial intelligence from 1950 to 2020.\n\nC. Discuss the ethical'