Fix test ImportError: use local Pydantic version detection instead of internal _compat

Replace `from fastapi._compat import PYDANTIC_V2` with local version
detection to avoid ImportError in redistribute/slim package tests where
fastapi._compat internals are not available.

Co-authored-by: Cursor <[email protected]>

tests/test_forms_fields_set.py

@@ -8,11 +8,13 @@ explicitly provided vs. which fields use defaults.
 
 from typing import Annotated
 
+import pydantic
 from fastapi import FastAPI, Form, Header, Query
-from fastapi._compat import PYDANTIC_V2
 from fastapi.testclient import TestClient
 from pydantic import BaseModel
 
+PYDANTIC_V2 = int(pydantic.VERSION.split(".")[0]) >= 2
+
 
 class FormModelFieldsSet(BaseModel):
     """Model for testing fields_set metadata preservation."""