admin_add_solver: invalid payload silently returns Ok(()) instead of a proper error

[codaMW]

Summary

In src/app/admin_add_solver.rs, when the incoming message has a missing or
wrong-typed payload, the handler logs an error but returns Ok(()):

let payload = if let Some(payload) = &inner_message.payload {
    payload
} else {
    error!("No pubkey found!");
    return Ok(());  // ← silent success on invalid input
};
let npubkey = if let Payload::TextMessage(p) = payload {
    p
} else {
    error!("No pubkey found!");
    return Ok(());  // ← same here
};

Impact

• The admin who sent the malformed request gets no feedback, no DM, no error
• The call site sees a successful result, making the failure invisible
• This is inconsistent with every other handler in src/app/ which returns
  Err(MostroCantDo(...)) or Err(MostroInternalErr(...)) for invalid input

Suggested Fix

Replace the silent Ok(()) returns with appropriate errors, for example:

} else {
    return Err(MostroInternalErr(ServiceError::InvalidPayload));
};

This would allow the caller to send a proper error DM to the admin and surface
the failure in logs with the correct severity level.

Related

• Consistent with how add_invoice_action and admin_take_dispute_action handle
  invalid input they return Err(...) rather than silently returning Ok(())

I wrote a failing test that reproduces this. Add to src/app/admin_add_solver.rs and run:

cargo test admin_add_solver_missing_payload

Output:

test app::admin_add_solver::tests::admin_add_solver_missing_payload_should_return_err_not_ok ... FAILED

BUG REPRODUCED: admin_add_solver_action returned Ok(()) on missing payload
instead of Err. The admin gets no feedback and the failure is invisible to the caller.