fix: validate ParentIDKey is non-empty in NewIndexer

[kuishou68]

Closes #952

Problem

In flow/indexer/parent/parent.go, NewIndexer validates Indexer, Transformer, and SubIDGenerator fields but silently accepts an empty ParentIDKey. When ParentIDKey is empty, Store() silently writes every parent document ID to the "" metadata key, making the parent-child relationship unresolvable for all consumers.

Fix

Added a validation check for config.ParentIDKey == "" in NewIndexer, consistent with the existing field validation pattern.

Testing

Passing empty ParentIDKey to NewIndexer now returns an error at construction time instead of silently corrupting data at runtime.

[CLAassistant]

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