Creates a new department in the company.

Business Rules:

• name: Required. Maximum 300 characters.
• type: Required. Must be a valid department type (e.g., DEPARTMENT).
• externalCode: Required. Maximum 50 characters. Must be unique within the company.
• description: Optional. Maximum 900 characters.
• costCenter: Optional. If provided, the cost center must exist in the company.

Validation:

• All required fields must be provided and not empty.
• The externalCode must not already exist for another department in the same company.
• If a costCenter is specified, it must be found by its externalCode.