Add changeset for filter operators patch release

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

.changeset/fix-filter-operators.md

@@ -0,0 +1,12 @@
+---
+"@objectstack/spec": patch
+"@objectstack/driver-memory": patch
+---
+
+Fix filter operators (contains, notContains, startsWith, endsWith, between, null) broken across spec and memory driver
+
+- Add `$notContains` to `StringOperatorSchema`, `FieldOperatorsSchema`, `FILTER_OPERATORS`, and `Filter` type
+- Add `notcontains` / `not_contains` to `VALID_AST_OPERATORS` and `AST_OPERATOR_MAP`
+- Fix memory driver `convertToMongoQuery()` passthrough to normalize non-standard operators to Mingo-compatible format
+- Add `$notContains` and `$null` operators to memory matcher
+- Fix undefined value guard in memory matcher to exclude `$exists`, `$ne`, and `$null`