feat: Add dynamic task ordering capability to Sequential and Hierarchical processes

- Add task_ordering_callback field to Crew class with proper validation
- Implement dynamic task selection in _execute_tasks method
- Add comprehensive error handling and validation for callback
- Include tests for various ordering scenarios and edge cases
- Maintain backward compatibility with existing code
- Support both task index and Task object returns from callback
- Add example demonstrating priority-based and conditional ordering

Fixes #3620

Co-Authored-By: João <joao@crewai.com>

examples/dynamic_task_ordering_example.py

@@ -0,0 +1,112 @@
+"""
+Example demonstrating dynamic task ordering in CrewAI.
+
+This example shows how to use the task_ordering_callback to dynamically
+determine the execution order of tasks based on runtime conditions.
+"""
+
+from crewai import Agent, Crew, Task
+from crewai.process import Process
+
+
+def priority_based_ordering(all_tasks, completed_outputs, current_index):
+    """
+    Order tasks by priority (lower number = higher priority).
+    
+    Args:
+        all_tasks: List of all tasks in the crew
+        completed_outputs: List of TaskOutput objects for completed tasks
+        current_index: Current task index (for default ordering)
+        
+    Returns:
+        int: Index of next task to execute
+        Task: Task object to execute next
+        None: Use default ordering
+    """
+    completed_task_ids = {output.task_id for output in completed_outputs}
+    
+    remaining_tasks = [
+        (i, task) for i, task in enumerate(all_tasks)
+        if task.id not in completed_task_ids
+    ]
+    
+    if not remaining_tasks:
+        return None
+    
+    remaining_tasks.sort(key=lambda x: getattr(x[1], 'priority', 999))
+    
+    return remaining_tasks[0][0]
+
+
+def conditional_ordering(all_tasks, completed_outputs, current_index):
+    """
+    Order tasks based on previous task outputs.
+    
+    This example shows how to make task ordering decisions based on
+    the results of previously completed tasks.
+    """
+    if len(completed_outputs) == 0:
+        return 0
+    
+    last_output = completed_outputs[-1]
+    
+    if "urgent" in last_output.raw.lower():
+        for i, task in enumerate(all_tasks):
+            if (hasattr(task, 'priority') and task.priority == 1 and 
+                task.id not in {out.task_id for out in completed_outputs}):
+                return i
+    
+    return None
+
+
+researcher = Agent(
+    role="Research Analyst",
+    goal="Gather and analyze information",
+    backstory="Expert at finding and synthesizing information"
+)
+
+writer = Agent(
+    role="Content Writer", 
+    goal="Create compelling content",
+    backstory="Skilled at crafting engaging narratives"
+)
+
+reviewer = Agent(
+    role="Quality Reviewer",
+    goal="Ensure content quality",
+    backstory="Meticulous attention to detail"
+)
+
+research_task = Task(
+    description="Research the latest trends in AI",
+    expected_output="Comprehensive research report",
+    agent=researcher
+)
+research_task.priority = 2
+
+urgent_task = Task(
+    description="Write urgent press release",
+    expected_output="Press release draft", 
+    agent=writer
+)
+urgent_task.priority = 1
+
+review_task = Task(
+    description="Review and edit content",
+    expected_output="Polished final content",
+    agent=reviewer
+)
+review_task.priority = 3
+
+crew = Crew(
+    agents=[researcher, writer, reviewer],
+    tasks=[research_task, urgent_task, review_task],
+    process=Process.sequential,
+    task_ordering_callback=priority_based_ordering,
+    verbose=True
+)
+
+if __name__ == "__main__":
+    print("Starting crew with dynamic task ordering...")
+    result = crew.kickoff()
+    print(f"Completed {len(result.tasks_output)} tasks")