mutation execution refactoring

[matej21]

This PR refines the execution flow of nested mutations in the Content API, focusing on improving determinism and simplifying the codebase. The key changes include:

• Elimination of async execution: Previous pseudo-async execution sometimes led to unpredictable and non-deterministic behavior in sub-operations.
• Clear separation of execution phases: The sub-operations execution flow is now explicitly divided into a pre-phase and post-phase, instead of relying on promise handling.
• Input order prioritization: Sub-operations are now executed based on the order in the input object, not by the schema’s field order. Although GraphQL treats input objects as unordered, this change allows for better control over execution order.