Reject empty source-ids in PartitionField / SortField

[kevinjqliu]

Inspired by the walrus issue in #3353

Problem

PartitionField and SortField accept the v3 source-ids list (added in #1554) and map it to the legacy singular source-id. Both validators try to reject an empty list:

if "source-id" not in data and (source_ids := data["source-ids"]):
    if isinstance(source_ids, list):
        if len(source_ids) == 0:
            raise ValueError("Empty source-ids is not allowed")
        ...
        data["source-id"] = source_ids[0]

The walrus uses truthiness, and [] is falsy — so the len(source_ids) == 0 branch is unreachable. Passing {"source-ids": []} silently skips the mapping, and Pydantic then reports a generic "field required" error instead of the intended message. A missing source-ids key also raises KeyError instead of being handled cleanly.

Fix

Replace the walrus with an explicit key check in both validators:

if "source-id" not in data and "source-ids" in data:
    source_ids = data["source-ids"]
    ...

This makes the empty-list validation reachable and avoids the KeyError.

Tests

Added regression tests that deserialize {"source-ids": []} and assert ValueError("Empty source-ids is not allowed") is raised, for both PartitionField and SortField.