Fix PostgresDbClient GetSimilarAsync minSimilarity requirement (#1057)

## Motivation and Context (Why the change? What's the scenario?)

Since #684 The PostgresDbClient will fail to return results that match
the minSimilarity requirement when multiple filters are used. This is
due to how the ```WHERE``` clause is prepared: ```filter1 OR filter2 OR
filter3 AND embedding <=> @Embedding < @maxDistance``` which cannot work
as expected since the `AND` operator takes precedence over the `OR`
operator
## High level description (Approach, Design)
Simply add parenthesis around the filters argument

Author: mrocha51248

extensions/Postgres/Postgres/Internals/PostgresDbClient.cs

@@ -416,13 +416,13 @@ DO UPDATE SET
 
         // Filtering logic, including filter by similarity
         filterSql = filterSql?.Trim().Replace(PostgresSchema.PlaceholdersTags, this._colTags, StringComparison.Ordinal);
-        if (string.IsNullOrWhiteSpace(filterSql))
-        {
-            filterSql = "TRUE";
-        }
 
         var maxDistance = 1 - minSimilarity;
-        filterSql += $" AND {this._colEmbedding} <=> @embedding < @maxDistance";
+
+        var distanceFilter = $"{this._colEmbedding} <=> @embedding < @maxDistance";
+        filterSql = string.IsNullOrWhiteSpace(filterSql)
+            ? distanceFilter
+            : $"({filterSql}) AND {distanceFilter}";
 
         if (sqlUserValues == null) { sqlUserValues = []; }