[Bugfix] Fix PlaceholderRange import error

[kyuyeunk]

Description

After upstream change in vllm-project/vllm#23779, tests/runner/jax/test_multimodal_manager.py test fails with below error. This PR fixes this issue.

==================================== ERRORS ====================================
--
  | _________ ERROR collecting tests/runner/jax/test_multimodal_manager.py _________
  | ImportError while importing test module '/workspace/tpu_commons/tests/runner/jax/test_multimodal_manager.py'.
  | Hint: make sure your test modules/packages have valid Python names.
  | Traceback:
  | /usr/local/lib/python3.12/importlib/__init__.py:90: in import_module
  | return _bootstrap._gcd_import(name[level:], package, level)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  | tests/runner/jax/test_multimodal_manager.py:12: in <module>
  | from vllm.v1.request import PlaceholderRange
  | E   ImportError: cannot import name 'PlaceholderRange' from 'vllm.v1.request' (/workspace/vllm/vllm/v1/request.py)

Tests

pytest tests/runner/jax/test_multimodal_manager.py

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.