Use exact four-character codes ('R499', 'D302') or description keywords
('professional', 'application'). Returns matching PSC entries with
code and description. Two-character families such as 'D3' can return
no autocomplete matches even when their descendant codes exist.
For partial PSC families or prefix expansion, use get_psc_filter_tree()
instead: path='Service/D/D3/' lists the D3 descendants; start with
path='Service/A/' to discover and drill into an AJ family.

Minimum 2 characters required. Single-character queries return first-N
alphabetical results from the upstream API (useless for matching) and
empty strings return HTTP 400.