fix: Address code review for schema-booleans parser

- Align NeverSchema validator stub with the spec: keyword is now "false"
  and the rejected value is passed through via fail() instead of require()
- Drop residual Map cast in parseAdditionalProperties; parse(value) accepts
  Object directly so no unchecked cast is needed
- Static-import assertThatThrownBy in SchemaParserTest for consistency
  with the existing assertThat static import
- Add parsesArrayWithBooleanFalseItems test covering items: false -> NeverSchema
- Add rejectsNullRawSchema test covering null raw input -> IllegalArgumentException

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thced

src/test/java/com/retailsvc/http/spec/schema/SchemaParserTest.java

@@ -427,4 +427,18 @@ void parsesArrayWithBooleanItemsSchema() {
     assertThat(s).isInstanceOf(ArraySchema.class);
     assertThat(((ArraySchema) s).items()).isInstanceOf(AlwaysSchema.class);
   }
+
+  @Test
+  void parsesArrayWithBooleanFalseItems() {
+    Schema s = SchemaParser.parse(Map.of("type", "array", "items", Boolean.FALSE));
+    assertThat(s).isInstanceOf(ArraySchema.class);
+    assertThat(((ArraySchema) s).items()).isInstanceOf(NeverSchema.class);
+  }
+
+  @Test
+  void rejectsNullRawSchema() {
+    assertThatThrownBy(() -> SchemaParser.parse(null))
+        .isInstanceOf(IllegalArgumentException.class)
+        .hasMessageContaining("schema must be a boolean or an object");
+  }
 }