Fix constrained grammar stop-token handling

[emilyhanyf]

Union tokenizer stop tokens with model-configured EOS tokens so grammar-constrained generation can terminate consistently with runtime stop detection.

Motivation

Modifications

Accuracy Tests

Speed Tests and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

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4. After green CI and required approvals, ask Merge Oncalls or people with Write permission to merge the PR.

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CI States

Latest PR Test (Base): ❌ Run #30252840626
Latest PR Test (Extra): ❌ Run #30252838665

[gemini-code-assist]

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