fix: #3590374 Validate elements machine names consistently

Closes #3590374 (closed)

AI-Generated: Yes (Used OpenCode to investigate the schema mismatch, extract shared validation logic, implement the constraint, and add focused kernel coverage. All changes were reviewed against the current 3.1.x source.)

Summary

• replace the incompatible machine_name schema type with a dedicated ECA constraint
• reuse FormFieldMachineName validation logic so forms and typed configuration stay consistent
• accept documented nested ][ names and tokenized names
• retain validation for invalid characters
• add focused kernel coverage for valid and invalid values

Testing

The focused test command is:

l3d ahoy test phpunitmodule eca --filter=EcaElementsMachineNameSchemaTest

Local execution is blocked in the current agent environment because access to /var/run/docker.sock is denied. The MR pipeline provides authoritative verification.