Add validation for copy_to non-existent fields when dynamic=false

[natea]

Summary

This PR fixes issue #112812 by adding validation to detect when copy_to targets a non-existent field when dynamic mappings are disabled (dynamic: false).

Problem

When using copy_to with a non-existent target field and dynamic mappings disabled, Elasticsearch would silently fail to copy the field value without any error or warning. This made it difficult to debug mapping issues.

Example

// Mapping with dynamic: false
{
  "dynamic": false,
  "properties": {
    "source_field": {
      "type": "text",
      "copy_to": "non_existent_field"  // This field doesn't exist!
    }
  }
}

// Before: Silently accepted, no data copied
// After: Throws IllegalArgumentException with clear error message

Solution

Added validation in FieldMapper.java to check if the target field exists when dynamic mappings are disabled. The validation provides a clear error message indicating which field is missing and why it cannot be created.

Changes

• FieldMapper.java: Added validation logic in parseCopyFields() method (lines 335-353)
• CopyToMapperTests.java: Added 3 comprehensive test methods covering various scenarios:
  • Test copying to non-existent field with dynamic=false (should fail)
  • Test copying to existing field with dynamic=false (should succeed)
  • Test that dynamic=true still works as before (auto-creates fields)

Error Message

When validation fails, users now get a clear error message:

Cannot copy to field [target_field] because it does not exist and dynamic mappings are disabled

Testing

• ✅ All existing copy_to tests pass
• ✅ Added 3 unit test methods covering edge cases
• ✅ Tests verify proper validation when dynamic=false
• ✅ Tests verify backward compatibility when dynamic=true

Related Issues

Fixes #112812

Checklist

• I have signed the Contributor License Agreement
• I have run the tests locally and they pass
• I have added tests that prove my fix is effective
• My changes follow the existing code style
• Error messages are clear and actionable

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)