[smg][ci] migrate validation tests to new infrastructure

[slin1237]

Migrate ignore_eos and large_max_new_tokens tests from e2e_grpc to chat_completions using the new pytest fixture infrastructure.

Tests migrated:

• TestIgnoreEOS.test_ignore_eos: Verifies ignore_eos=True continues generation beyond EOS token until max_tokens is reached
• TestLargeMaxNewTokens.test_concurrent_chat_completions: Verifies concurrent requests with large token generation complete successfully

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.

Review Process

1. Ping Merge Oncalls to start the PR flow. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments (/tag-run-ci-label, /rerun-failed-ci, /tag-and-rerun-ci) or contact authorized users to do so.
4. After green CI and required approvals, ask Merge Oncalls to merge.

[gemini-code-assist]

Summary of Changes

Hello @slin1237, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors existing end-to-end validation tests for the SGLang model gateway. The primary goal is to transition these tests to a more modern and maintainable pytest fixture-based infrastructure, specifically for chat_completions. This change streamlines the testing process and aligns with the project's evolving testing standards, ensuring continued reliability of core features like ignore_eos and concurrent request handling.

Highlights

• Test Infrastructure Migration: Validation tests for ignore_eos and large_max_new_tokens have been migrated from the older e2e_grpc framework to a new chat_completions based infrastructure utilizing pytest fixtures.
• Specific Tests Migrated: The TestIgnoreEOS.test_ignore_eos test, which verifies generation beyond EOS tokens when ignore_eos=True, and TestLargeMaxNewTokens.test_concurrent_chat_completions, which checks concurrent requests with large token generation, are now part of the new infrastructure.
• New Test File: A new test file, sgl-model-gateway/e2e_test/chat_completions/test_validation.py, has been added to house these refactored validation tests.

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[gemini-code-assist]

Code Review

This pull request successfully migrates two validation tests, test_ignore_eos and test_concurrent_chat_completions, to the new pytest fixture-based infrastructure. The new test file is well-organized and the logic from the original tests has been preserved. I've provided a few suggestions to improve the code's robustness and maintainability, including making the tokenizer cache thread-safe, removing unused variables, and refactoring duplicated code.