chore: add changeset for rest 4xx error mapping

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang

.changeset/rest-map-schema-errors.md

@@ -0,0 +1,15 @@
+---
+'@objectstack/rest': patch
+---
+
+fix(rest): map schema-mismatch & not-null driver errors to structured 4xx
+
+`mapDataError` collapsed any SQL-looking driver error into a generic
+`500 DATABASE_ERROR`, so a bad write payload to the data API leaked a 500
+instead of a fixable 4xx (e.g. `POST /data/sys_team` with an unknown field,
+or omitting a required column). It now maps unknown-column errors to
+`400 INVALID_FIELD { field }` and not-null violations to
+`400 VALIDATION_FAILED { fields:[{required}] }` across SQLite/Postgres/MySQL
+phrasings, placed before the unknown-object branch so Postgres
+`column … of relation … does not exist` is not mis-mapped to 404. Genuine
+driver faults still return 500; unique violations still return 409.