Fix group precedence in MilvusFilterExpressionConverter

The `doGroup` override converted a parenthesized filter group into
`content && content`, duplicating the group body and dropping the
parentheses that expressed the grouping. Because Milvus binds `&&`
tighter than `||`, a filter such as

    (year >= 2020 OR country == 'BG') AND city != 'Sofia'

was emitted without parentheses, so the trailing `AND` no longer
applied to the whole group and the query silently matched the wrong
documents.

Replace the override with the conventional `doStartGroup`/`doEndGroup`
hooks that wrap the group content in parentheses, consistent with the
other filter expression converters such as
`PgVectorFilterExpressionConverter`. Update the affected test, which
asserted the incorrect output, and add a test covering a group used as
the right-hand operand.

Closes #6543

Signed-off-by: Seol-JY <70826982+Seol-JY@users.noreply.github.com>

Author: Seol-JY

vector-stores/spring-ai-milvus-store/src/main/java/org/springframework/ai/vectorstore/milvus/MilvusFilterExpressionConverter.java

@@ -17,7 +17,6 @@
 package org.springframework.ai.vectorstore.milvus;
 
 import org.springframework.ai.vectorstore.filter.Filter.Expression;
-import org.springframework.ai.vectorstore.filter.Filter.ExpressionType;
 import org.springframework.ai.vectorstore.filter.Filter.Group;
 import org.springframework.ai.vectorstore.filter.Filter.Key;
 import org.springframework.ai.vectorstore.filter.converter.AbstractFilterExpressionConverter;
@@ -70,8 +69,13 @@ private String getOperationSymbol(Expression exp) {
 	}
 
 	@Override
-	protected void doGroup(Group group, StringBuilder context) {
-		this.convertOperand(new Expression(ExpressionType.AND, group.content(), group.content()), context); // trick
+	protected void doStartGroup(Group group, StringBuilder context) {
+		context.append("(");
+	}
+
+	@Override
+	protected void doEndGroup(Group group, StringBuilder context) {
+		context.append(")");
 	}
 
 	@Override

vector-stores/spring-ai-milvus-store/src/test/java/org/springframework/ai/vectorstore/milvus/MilvusFilterExpressionConverterTests.java

@@ -98,7 +98,18 @@ public void testGroup() {
 						new Expression(EQ, new Key("country"), new Value("BG")))),
 				new Expression(NIN, new Key("city"), new Value(List.of("Sofia", "Plovdiv")))));
 		assertThat(vectorExpr).isEqualTo(
-				"metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\" && metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\" && metadata[\"city\"] not in [\"Sofia\",\"Plovdiv\"]");
+				"(metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\") && metadata[\"city\"] not in [\"Sofia\",\"Plovdiv\"]");
+	}
+
+	@Test
+	public void testGroupAsRightOperand() {
+		// city == "Sofia" AND (year >= 2020 OR country == "BG")
+		String vectorExpr = this.converter
+			.convertExpression(new Expression(AND, new Expression(EQ, new Key("city"), new Value("Sofia")),
+					new Group(new Expression(OR, new Expression(GTE, new Key("year"), new Value(2020)),
+							new Expression(EQ, new Key("country"), new Value("BG"))))));
+		assertThat(vectorExpr).isEqualTo(
+				"metadata[\"city\"] == \"Sofia\" && (metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\")");
 	}
 
 	@Test