[Bugfix] Lower gpt-oss max cudagraph size to 992 to be compatible with FA3

[mgoin]

Purpose

Due to compiling full cudagraphs by default now in #25444, gpt-oss crashes on Hopper.

vllm serve openai/gpt-oss-20b
...
(EngineCore_DP0 pid=2061997)   File "/home/mgoin/code/vllm/vllm/v1/worker/gpu_model_runner.py", line 3503, in create_attn_groups
(EngineCore_DP0 pid=2061997)     attn_metadata_builders.append(attn_backend.get_builder_cls()(
(EngineCore_DP0 pid=2061997)                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=2061997)   File "/home/mgoin/code/vllm/vllm/v1/attention/backends/flash_attn.py", line 204, in __init__
(EngineCore_DP0 pid=2061997)     raise ValueError(
(EngineCore_DP0 pid=2061997) ValueError: Capture size larger than 992 is not supported for full cuda graph.

Test Plan

Test Result

vllm serve openai/gpt-oss-20b crashes on main but succeeds with this PR

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Essential Elements of an Effective PR Description Checklist

• 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.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly addresses a crash on Hopper GPUs for the gpt-oss model by lowering the maximum CUDA graph capture size to 992, which is the compatibility limit for FlashAttention v3. The change is well-targeted and effective. I have one suggestion to improve the code's maintainability by replacing the hardcoded value 992 with a named constant.