- Implicitly fixes invoke_context.return_data not being reset between instructions in process_message. (#21671) (#21684)

- Lets InvokeContext::process_cross_program_instruction() handle the first invocation depth too.
- Marks InvokeContext::verify(), InvokeContext::verify_and_update() and InvokeContext::process_executable_chain() private.
- Renames InvokeContext::process_cross_program_instruction() to InvokeContext::process_instruction().
- Removes InvokeContext::new_mock_with_sysvars().

(cherry picked from commit 1df88837c8)

Co-authored-by: Alexander Meißner <AlexanderMeissner@gmx.net>

programs/stake/src/stake_instruction.rs

@@ -437,7 +437,7 @@ mod tests {
             &preparation.message,
             &preparation.message.instructions[0],
             &program_indices,
-            Some(&preparation.account_indices),
+            &preparation.account_indices,
         )?;
         super::process_instruction(1, &instruction.data, &mut invoke_context)
     }
@@ -1084,7 +1084,7 @@ mod tests {
                 &preparation.message,
                 &preparation.message.instructions[0],
                 &program_indices,
-                Some(&preparation.account_indices),
+                &preparation.account_indices,
             )
             .unwrap();
         assert_eq!(