fix: correct aggregation error message case and aget_routes signature

[totto]

Changes

1. Aggregation error message uses wrong case (routers/base.py)

The validation checks against lowercase values but the error message told users to use uppercase:

if self.aggregation not in ["sum", "mean", "max"]:  # checks lowercase
    raise ValueError(
        f"... Choose either 'SUM', 'MEAN', or 'MAX'."  # ← uppercase
    )

Following the error message causes the same error to trigger again. Fixed by aligning the message to lowercase. Applied to both occurrences (lines 424 and 1533 before this change).

2. aget_routes in LocalIndex declared as sync (index/local.py)

aget_routes follows the a-prefix async convention used throughout the codebase (e.g. aadd, adelete, asearch) but was declared as a sync def. The error message also said "Sync remove is not implemented" — wrong method name and wrong type.

Fixed: async def aget_routes with correct error message.

3. Dead code after raise NotImplementedError in _refresh_routes

13 lines of unreachable code (incomplete draft implementation) were left after the raise NotImplementedError. Removed.

[jamescalam]

great catch, merging now thanks for the contribution!