Fix Pydantic class-based config deprecation warnings (#9279)

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/server/session/conversation_init_data.py

@@ -1,4 +1,4 @@
-from pydantic import Field
+from pydantic import ConfigDict, Field
 
 from openhands.integrations.provider import CUSTOM_SECRETS_TYPE, PROVIDER_TOKEN_TYPE
 from openhands.integrations.service_types import ProviderType
@@ -18,6 +18,6 @@ class ConversationInitData(Settings):
     conversation_instructions: str | None = Field(default=None)
     git_provider: ProviderType | None = Field(default=None)
 
-    model_config = {
-        'arbitrary_types_allowed': True,
-    }
+    model_config = ConfigDict(
+        arbitrary_types_allowed=True,
+    )