Consider supporting single-element logical filter nodes

Context

When building attribute filters programmatically, it's common to collect conditions into a list and then wrap them in {"type": "and", "filters": [...]}. This works well for 2+ conditions, but requires special-casing for 0 and 1 conditions since ChromaDB rejects $and with a single entry.

This leads to boilerplate like:

if len(filters) == 0:
    attributes_filter = None
elif len(filters) == 1:
    attributes_filter = filters[0]
else:
    attributes_filter = {"type": "and", "filters": filters}

It would be nice if raghilda could unwrap single-element logical nodes automatically during filter compilation (before emitting to the backend), so users could always write:

attributes_filter = {"type": "and", "filters": filters} if filters else None

Possible approach

In _emit_chroma_where (and similar emitters), if a logical node has exactly one child, emit the child directly instead of wrapping it in $and/$or. The DuckDB and OpenAI emitters may already handle this gracefully, so it might only need a ChromaDB-specific change.

Start in _emit_chroma_where and compare its behavior with the DuckDB and OpenAI emitters. Confirm that a logical node with one child emits the child directly while existing multi-child behavior remains unchanged; run the project’s tests to verify the filter compilation behavior.