[Core] add deprecation warning for pydantic v1

[abrarsheikh]

related to #58876

❯ python -c "
import ray
# Temporarily patch to test the warning shows
from ray._common import pydantic_compat
original = pydantic_compat.IS_PYDANTIC_2
pydantic_compat.IS_PYDANTIC_2 = False  # Simulate Pydantic v1

ray.init()
ray.shutdown()

pydantic_compat.IS_PYDANTIC_2 = original
"
2025-12-26 22:33:01,387 INFO worker.py:1811 -- Connecting to existing Ray cluster at address: 172.31.7.228:6379...
2025-12-26 22:33:01,407 INFO worker.py:1991 -- Connected to Ray cluster. View the dashboard at 127.0.0.1:8265 
/home/ubuntu/ray/python/ray/_private/worker.py:2039: FutureWarning: Tip: In future versions of Ray, Ray will no longer override accelerator visible devices env var if num_gpus=0 or num_gpus=None (default). To enable this behavior and turn off this error message, set RAY_ACCEL_ENV_VAR_OVERRIDE_ON_ZERO=0
  warnings.warn(
/home/ubuntu/ray/python/ray/_private/worker.py:2050: FutureWarning: Pydantic v1 is deprecated and will no longer be supported in Ray 2.56. Please upgrade to Pydantic v2 by running `pip install -U pydantic`. See https://github.com/ray-project/ray/issues/58876 for more details.
  warnings.warn(
 ~/ray │ on 58876-abrar-pydantic *91       
"

[gemini-code-assist]

Code Review

This pull request adds a deprecation warning for Pydantic v1, which is displayed during ray.init(). The implementation is correct, using log_once to show the warning only once. The warning message is clear, providing users with instructions to upgrade and a link to the relevant issue for more details. The change is well-placed and follows the existing coding style of the file. Overall, this is a solid and helpful improvement.