Update typing of ga and gs id validation method

gso/utils/types/geant_ids.py

 """Type definitions for the GA and GS IDs."""
 
 from functools import partial
-from typing import Annotated
+from typing import Annotated, Literal
 
 from pydantic import AfterValidator
 
 from gso.services.subscriptions import is_resource_type_value_unique
 
 
-def validate_id(value: str, prefix: str, field_name: str) -> str:
+def validate_id(value: str, prefix: Literal["GA", "GS"], field_name: str) -> str:
     """Validate that the ID is unique, has the correct prefix, and is within valid constraints.
 
     Args:
-        value (str): The ID value to validate.
-        prefix (str): The required prefix for the ID (e.g., "GA-" or "GS-").
-        field_name (str): The database field name to check for uniqueness (e.g., "ga_id" or "gs_id").
+        value: The ID value to validate.
+        prefix: The required prefix for the ID.
+        field_name: The database field name to check for uniqueness.
 
     Raises:
         ValueError: If the ID is not valid.
@@ -29,21 +29,21 @@ def validate_id(value: str, prefix: str, field_name: str) -> str:
         raise ValueError(err)
 
     try:
-        numeric_part = int(value[len(prefix) :])
+        numeric_part = int(value.split("-")[-1])
     except ValueError:
         err = f"{field_name} must have a numeric part after the prefix '{prefix}'."
         raise ValueError(err) from ValueError
 
     if min_range <= numeric_part <= max_range:
-        err = f"{field_name} must not have a numeric part between 50000 and 99999."
+        err = f"{field_name} must not have a numeric part between 50000 and 99999, received {numeric_part}"
         raise ValueError(err)
 
     if not is_resource_type_value_unique(field_name, value):
-        err = f"{field_name} must be unique."
+        err = f"{field_name} must be unique, {value} is already in use."
         raise ValueError(err)
 
     return value
 
 
-IMPORTED_GA_ID = Annotated[str, AfterValidator(partial(validate_id, prefix="GA-", field_name="ga_id"))]
-IMPORTED_GS_ID = Annotated[str, AfterValidator(partial(validate_id, prefix="GS-", field_name="gs_id"))]
+IMPORTED_GA_ID = Annotated[str, AfterValidator(partial(validate_id, prefix="GA", field_name="ga_id"))]
+IMPORTED_GS_ID = Annotated[str, AfterValidator(partial(validate_id, prefix="GS", field_name="gs_id"))]