Detect copy_to non-existent field when dynamic mappings disabled (#112812)

This change adds validation to detect when copy_to targets a non-existent
field when dynamic mappings are disabled (dynamic=false). Previously, this
configuration would silently fail to copy values at runtime.

Changes:
- Added validation in FieldMapper.validate() to check if copy_to target
  fields exist when dynamic=false
- Throws IllegalArgumentException with clear error message when validation fails
- Added comprehensive unit tests covering the new validation logic

The validation only applies when dynamic=false since other dynamic settings
(true, strict, runtime) have different behaviors for handling unknown fields.

Closes #112812

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: natea

server/src/main/java/org/elasticsearch/index/mapper/FieldMapper.java

@@ -332,6 +332,10 @@ public final void validate(MappingLookup mappers) {
                 throw new IllegalArgumentException("[copy_to] may not be used to copy from a multi-field: [" + this.fullPath() + "]");
             }
 
+            // Check if dynamic mappings are disabled
+            ObjectMapper.Dynamic rootDynamic = ObjectMapper.Dynamic.getRootDynamic(mappers);
+            boolean isDynamicDisabled = rootDynamic == ObjectMapper.Dynamic.FALSE;
+
             final String sourceScope = mappers.nestedLookup().getNestedParent(this.fullPath());
             for (String copyTo : this.copyTo().copyToFields()) {
                 if (mappers.isMultiField(copyTo)) {
@@ -341,6 +345,13 @@ public final void validate(MappingLookup mappers) {
                     throw new IllegalArgumentException("Cannot copy to field [" + copyTo + "] since it is mapped as an object");
                 }
 
+                // When dynamic is false, check if the target field exists
+                if (isDynamicDisabled && mappers.getMapper(copyTo) == null) {
+                    throw new IllegalArgumentException(
+                        "Cannot copy to field [" + copyTo + "] because it does not exist and dynamic mappings are disabled"
+                    );
+                }
+
                 final String targetScope = mappers.nestedLookup().getNestedParent(copyTo);
                 checkNestedScopeCompatibility(sourceScope, targetScope);
             }

server/src/test/java/org/elasticsearch/index/mapper/CopyToMapperTests.java

@@ -32,6 +32,62 @@
 
 public class CopyToMapperTests extends MapperServiceTestCase {
 
+    public void testCopyToNonExistentFieldWithDynamicFalse() {
+        Exception e = expectThrows(IllegalArgumentException.class, () -> createDocumentMapper(topMapping(b -> {
+            b.field("dynamic", false);
+            b.startObject("properties");
+            {
+                b.startObject("test_field");
+                {
+                    b.field("type", "text");
+                    b.field("copy_to", "missing_field");
+                }
+                b.endObject();
+            }
+            b.endObject();
+        })));
+        assertThat(
+            e.getMessage(),
+            equalTo("Cannot copy to field [missing_field] because it does not exist and dynamic mappings are disabled")
+        );
+    }
+
+    public void testCopyToExistingFieldWithDynamicFalse() throws Exception {
+        // This should succeed as the target field exists
+        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
+            b.field("dynamic", false);
+            b.startObject("properties");
+            {
+                b.startObject("test_field");
+                {
+                    b.field("type", "text");
+                    b.field("copy_to", "target_field");
+                }
+                b.endObject();
+                b.startObject("target_field");
+                {
+                    b.field("type", "text");
+                }
+                b.endObject();
+            }
+            b.endObject();
+        }));
+        assertNotNull(mapper);
+    }
+
+    public void testCopyToNonExistentFieldWithDynamicTrue() throws Exception {
+        // This should succeed as dynamic is true (default)
+        MapperService mapperService = createMapperService(mapping(b -> {
+            b.startObject("test_field");
+            {
+                b.field("type", "text");
+                b.field("copy_to", "missing_field");
+            }
+            b.endObject();
+        }));
+        assertNotNull(mapperService.documentMapper());
+    }
+
     @SuppressWarnings("unchecked")
     public void testCopyToFieldsParsing() throws Exception {