fix: validate where-operators and align the in-memory matcher to the documented set

`find({ where })` had two gaps that could silently return nothing. The in-memory
matcher (`matchesQuery`, used for egress re-validation and historical reads) was
missing several documented operators — `in`, `greaterThanOrEqual`,
`lessThanOrEqual` — so it disagreed with the index path, which does handle them.
And an unknown operator key (a typo, or `notIn` written where `not: { in }` was
meant) fell through to a nested-object-field interpretation and matched nothing.

- Align `matchesQuery` to the full documented operator set (add the missing
  aliases; the default branch now throws instead of treating an unknown key as a
  nested field — dotted paths remain the supported nested form).
- Validate the `where` filter up front (`validateWhereFilter`), throwing a typed
  `BrainyError('INVALID_QUERY')` that names the unknown operator, recursing into
  `allOf` / `anyOf` / `not`.
- Stop excluding a user field literally named `level` from the metadata index
  (it collided with an internal never-index key), so numeric/exact filters on it
  resolve instead of returning 0.

Adds unit coverage for the operator set and the throw-on-unknown behavior.

src/utils/metadataIndex.ts

@@ -1166,8 +1166,17 @@ export class MetadataIndexManager implements MetadataIndexProvider {
   private extractIndexableFields(data: any): Array<{ field: string, value: any }> {
     const fields: Array<{ field: string, value: any }> = []
 
-    // Fields that should NEVER be indexed (vectors, embeddings, large arrays, HNSW internals)
-    const NEVER_INDEX = new Set(['vector', 'embedding', 'embeddings', 'connections', 'level', 'id'])
+    // Fields that should NEVER be indexed: bulk structural payloads that would
+    // blow up the index (the 384-dim vector, embeddings, the adjacency list).
+    // These are also caught by the array-size guard below, but naming them is
+    // belt-and-suspenders. NOTE: `level` was previously here (an HNSW node's
+    // layer) but it never actually reaches this path — every caller passes a
+    // metadata bag or Entity record, neither of which carries the node's
+    // `level` — so its only effect was to silently drop a legitimate USER
+    // metadata field named `level` (log level, skill level, access level…),
+    // making `where: { level: … }` return nothing. Removed. (`id` stays: it is
+    // the reserved entity-identity field, resolved specially by find().)
+    const NEVER_INDEX = new Set(['vector', 'embedding', 'embeddings', 'connections', 'id'])
 
     const extract = (obj: any, prefix = ''): void => {
       for (const [key, value] of Object.entries(obj)) {
@@ -1278,7 +1287,10 @@ export class MetadataIndexManager implements MetadataIndexProvider {
       return data.map(d => this.extractTextContent(d)).filter(Boolean).join(' ')
     }
     if (typeof data === 'object') {
-      const skipKeys = new Set(['vector', 'embedding', 'embeddings', 'connections', 'level', 'id'])
+      // Mirror of NEVER_INDEX for the text-extraction path: bulk structural
+      // payloads only. `level` removed for the same reason (it silently dropped
+      // a real user field from hybrid text search too).
+      const skipKeys = new Set(['vector', 'embedding', 'embeddings', 'connections', 'id'])
       const texts: string[] = []
       for (const [key, value] of Object.entries(data)) {
         // Skip internal fields and numeric keys (array indices)