docs: add missing 409 error case for unknown sender ID(name)

[AganzeFelicite]

This PR updates the SMS API error documentation to include a previously undocumented 409 error case:

response body = {
  "error": "Unknown sender id  E****",
  "message": "Unknown sender id E*****",
  "status": 409
}

Code	Text	Meaning
409	conflict	Unknown sender ID – the sender ID used is not recognized or not registered with the SMS provider, even if the token is correct

Reason for the change:

• The API actually returns this 409 response when an unregistered sender ID is used.
• Helps developers diagnose SMS sending failures related to sender ID misconfiguration.
• Complements existing documented 409 cases (unsupported country, unsupported telco, wrong phone number format).

No functional changes were made to the API; this is purely documentation.