Add NOT and multi-word capability to item attribute search #4654

Summary

Extend the attribute search syntax introduced in #4652 with three capabilities:

1. NOT operator (!) — prefix an attribute value with ! to negate the match.

   • Example: Color:!Blue returns all results where attribute Color is not Blue.

2. Multi-word value matching (quoted strings) — wrap a multi-word value in double quotes so it is treated as a single value instead of being split into separate search terms.

   • Example: Color:"Sky Blue" returns all results where attribute Color is Sky Blue, instead of treating Sky and Blue as separate search criteria.

3. Multi-word attribute name matching (quoted names) — wrap a multi-word attribute name in double quotes so it is matched against attribute definition names containing spaces.

   • Example: "Sale Type":Blue searches for an attribute definition named Sale Type with value Blue.

These should also compose, e.g. "Sale Type":!"Sky Blue" to exclude a multi-word value for a multi-word attribute name.

Reference

Attribute search feature introduced in #4652.