Function transition 

pub fn transition(
    doc: &mut Document,
    workflow: &WorkflowDefinition,
    target_phase_id: &str,
) -> Result<(), WorkflowError>

Attempts to transition a document from its current phase to a specified target phase according to the rules defined in the provided workflow.

If the transition is valid, the document.current_phase field is updated in place to the target_phase_id.

This function performs several checks:

1. Verifies that the document’s workflow_id matches the provided workflow.id().
2. Ensures the target_phase_id actually exists within the workflow definition.
3. Handles initial transitions for new documents (those with an empty current_phase), only allowing them to transition to the workflow’s designated “Start” phase.
4. Validates that a direct transition path exists from the document’s current_phase to the target_phase_id within the workflow graph.

Arguments

• doc - A mutable reference to the Document to be transitioned.
• workflow - The WorkflowDefinition that defines the valid phases and transitions for this document.
• target_phase_id - The id of the phase to which the document should transition.

Returns

• Ok(()) if the transition was successful and the document’s phase was updated.
• Err(WorkflowError) if any of the validation checks fail (e.g., workflow mismatch, unknown phase, invalid transition, or no current phase on a non-new document).