[BUG] ImportError risk: run_in_threadpool from starlette.concurrency may not be available in all environments

[Binilkks]

Describe the bug
The code imports run_in_threadpool from starlette.concurrency, but this import may fail or be unavailable in some environments or with older Starlette versions.

To Reproduce
Steps to reproduce the behavior:
Use a version of Starlette that does not provide run_in_threadpool.
Run the code or tests.
Observe an ImportError at startup or during execution.

Expected behavior
The code should either:

Check for the presence of run_in_threadpool and provide a fallback, or
Clearly document the minimum required Starlette version in requirements and documentation.

Screenshots
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Environment:

• OS: macOS
• FastAPI Utils version : 0.8.0
  Starlette version: 0.50.0

import fastapi_utils
import fastapi
import pydantic.utils
print(fastapi_utils.__version__)
print(fastapi.__version__)
print(pydantic.utils.version_info())

• Python version, get it with:

python --version

Additional context
Add any other context about the problem here.