refactor: move PKCE pair generatio for airtable

- Removed the `generate_pkce_pair` function from `airtable_add_connector_route.py` and relocated it to `oauth_security.py` for better organization.
- Updated imports in `airtable_add_connector_route.py` to reflect the new location of the PKCE generation function.

surfsense_backend/app/routes/airtable_add_connector_route.py

@@ -1,7 +1,5 @@
 import base64
-import hashlib
 import logging
-import secrets
 from datetime import UTC, datetime, timedelta
 from uuid import UUID
 
@@ -26,7 +24,7 @@ from app.utils.connector_naming import (
     check_duplicate_connector,
     generate_unique_connector_name,
 )
-from app.utils.oauth_security import OAuthStateManager, TokenEncryption
+from app.utils.oauth_security import OAuthStateManager, TokenEncryption, generate_pkce_pair
 
 logger = logging.getLogger(__name__)
 
@@ -75,28 +73,6 @@ def make_basic_auth_header(client_id: str, client_secret: str) -> str:
     return f"Basic {b64}"
 
 
-def generate_pkce_pair() -> tuple[str, str]:
-    """
-    Generate PKCE code verifier and code challenge.
-
-    Returns:
-        Tuple of (code_verifier, code_challenge)
-    """
-    # Generate code verifier (43-128 characters)
-    code_verifier = (
-        base64.urlsafe_b64encode(secrets.token_bytes(32)).decode("utf-8").rstrip("=")
-    )
-
-    # Generate code challenge (SHA256 hash of verifier, base64url encoded)
-    code_challenge = (
-        base64.urlsafe_b64encode(hashlib.sha256(code_verifier.encode("utf-8")).digest())
-        .decode("utf-8")
-        .rstrip("=")
-    )
-
-    return code_verifier, code_challenge
-
-
 @router.get("/auth/airtable/connector/add")
 async def connect_airtable(space_id: int, user: User = Depends(current_active_user)):
     """

surfsense_backend/app/utils/oauth_security.py

@@ -29,6 +29,17 @@ def generate_code_verifier(length: int = 128) -> str:
     return "".join(_PKCE_RNG.choice(_PKCE_CHARS) for _ in range(length))
 
 
+def generate_pkce_pair(length: int = 128) -> tuple[str, str]:
+    """Generate a PKCE code_verifier and its S256 code_challenge."""
+    verifier = generate_code_verifier(length)
+    challenge = (
+        base64.urlsafe_b64encode(hashlib.sha256(verifier.encode()).digest())
+        .decode()
+        .rstrip("=")
+    )
+    return verifier, challenge
+
+
 class OAuthStateManager:
     """Manages secure OAuth state parameters with HMAC signatures."""