Updates a role identified by externalCode in the URL.

Business rules:

• name, similarity, and externalCode are required and must not be empty.
• jobLevel and description must be sent on every request; use null or "" to clear optional values in EO.
• externalCode in the body must be unique within the company (cannot match another role's code).

Errors (HTTP):

• 404 when no role matches the URL externalCode for the company.
• 409 when the new externalCode is already used by another role.
• 400 for validation failures (invalid enums, missing keys, empty required strings).