[CI] Fix Plugin Tests Tests

[robertgshaw2-redhat]

Purpose

• recent PR [DCP] Support dcp kv_cache interleave size > 1 #26696 broke Plugin tests (2GPUs)
• https://buildkite.com/vllm/ci/builds/38270#019a6e4d-82cd-49fe-9415-ec8972e02f85

Test Plan

• CI (validated locally)

uv pip install -e ./plugins/vllm_add_dummy_platform
pytest -v -s plugins_tests/test_platform_plugins.py

previously failed on this assert (block size was null)

Test Result

now passes

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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[LucasWilkinson]

LGTM thanks!

[gemini-code-assist]

Code Review

This pull request correctly fixes a crash in plugin tests by ensuring an assertion related to Decode Context Parallelism (DCP) is only executed when DCP is active. This prevents a TypeError when block_size is None in non-DCP scenarios. I've added one suggestion to further improve robustness by adding an explicit check for block_size when DCP is enabled, which will provide a more informative error message in case of a misconfiguration.

[gemini-code-assist]

While this change correctly guards the assertion for when Decode Context Parallelism (DCP) is enabled, it could be made more robust. If decode_context_parallel_size > 1 but self.cache_config.block_size is None (e.g., due to a misconfigured custom platform plugin), the assertion will raise an unhelpful TypeError. It would be better to add an explicit check for block_size to provide a more informative error message.

        if self.parallel_config.decode_context_parallel_size > 1:
            assert self.cache_config.block_size is not None, (
                "block_size must be set when using decode context parallelism (DCP)."
            )
            assert (
                self.parallel_config.dcp_kv_cache_interleave_size
                <= self.cache_config.block_size
                and self.cache_config.block_size
                % self.parallel_config.dcp_kv_cache_interleave_size
                == 0
            ), (
                f"Block_size({self.cache_config.block_size}) should be "
                "greater than or equal to and divisible by dcp_kv_cache_interleave_size "
                f"({self.parallel_config.dcp_kv_cache_interleave_size})."
            )