[BugFix] Fix lazy imports involving outlines_core

[22quinn]

Purpose

Fix outlines_core lazy imports broken by #26633

Annotations are executed at import time by default. To reproduce the issue, run the following

uv pip uninstall outlines_core
python -c "from vllm.v1.engine.llm_engine import LLMEngine"

Error stack trace

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/engine/llm_engine.py", line 31, in <module>
    from vllm.v1.engine.core_client import EngineCoreClient
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/engine/core_client.py", line 42, in <module>
    from vllm.v1.engine.core import EngineCore, EngineCoreProc
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/engine/core.py", line 39, in <module>
    from vllm.v1.core.kv_cache_utils import (
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/core/kv_cache_utils.py", line 27, in <module>
    from vllm.v1.request import Request
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/request.py", line 22, in <module>
    from vllm.v1.structured_output.request import StructuredOutputRequest
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/structured_output/__init__.py", line 17, in <module>
    from vllm.v1.structured_output.backend_xgrammar import XgrammarBackend
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/structured_output/backend_xgrammar.py", line 20, in <module>
    from vllm.v1.structured_output.utils import (
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/structured_output/utils.py", line 120, in <module>
    class OutlinesVocabulary:
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/v1/structured_output/utils.py", line 126, in OutlinesVocabulary
    def __init__(self, vocabulary: oc.Vocabulary) -> None:
                                   ^^^^^^^^^^^^^
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/utils/import_utils.py", line 320, in __getattr__
    self._module = self._load()
                   ^^^^^^^^^^^^
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/utils/import_utils.py", line 310, in _load
    raise err from None
  File "/data/users/quinnzhu/gitrepos/vllm/vllm/utils/import_utils.py", line 304, in _load
    module = importlib.import_module(self.__name__)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ModuleNotFoundError: No module named 'outlines_core'

Test Plan

uv pip uninstall outlines_core
python -c "from vllm.v1.engine.llm_engine import LLMEngine"

Test Result

Passed

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[gemini-code-assist]

Code Review

This pull request addresses a bug where lazy imports for outlines_core would fail if the package was not installed. The root cause was the evaluation of type annotations at import time, which triggered the lazy loader. The fix correctly introduces from __future__ import annotations in the affected files (vllm/v1/structured_output/backend_outlines.py and vllm/v1/structured_output/utils.py) to defer the evaluation of type hints. This change is idiomatic, well-targeted, and effectively resolves the ModuleNotFoundError. The implementation is clean and I have no further suggestions.