[V1][Spec Decode] Eagle unit tests

[wwl2755]

This PR adds comprehensive unit tests for the EagleProposer class, including model loading, token proposal generation, and input preparation for both Eagle and Eagle3 variants.

With these tests in place, developers can more easily verify and tune the functionality of the speculative decoding implementation.

Any comment or guidance are appreciated! @LiuXiaoxuanPKU @WoosukKwon

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @wwl2755.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[zixi-qi]

LGTM!

[wwl2755]

Hi @zixi-qi , I have just rebased on main and fix the tests for new commits accordingly. PTAL. Thank you!

cc: @WoosukKwon @LiuXiaoxuanPKU

[zixi-qi]

Hi @zixi-qi , I have just rebased on main and fix the tests for new commits accordingly. PTAL. Thank you!

cc: @WoosukKwon @LiuXiaoxuanPKU

Thanks for the update! Changes LGTM

[WoosukKwon]

LGTM! Sorry for the late review 🙏