Fix Flow initialization with Pydantic models having required fields

- Modified _create_initial_state() to accept kwargs parameter
- Pass kwargs when instantiating BaseModel classes
- Updated __init__() to pass kwargs to _create_initial_state()
- Added comprehensive tests covering various scenarios

Fixes #3744

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

tests/test_flow_pydantic_required_fields.py

@@ -0,0 +1,145 @@
+"""Tests for Flow initialization with Pydantic models having required fields.
+Covers https://github.com/crewAIInc/crewAI/issues/3744
+"""
+
+import pytest
+from pydantic import BaseModel, ValidationError
+
+from crewai.flow.flow import Flow, FlowState, listen, start
+
+
+class RequiredState(BaseModel):
+    """State model with required fields."""
+    name: str
+    age: int
+
+
+class RequiredStateFlow(Flow[RequiredState]):
+    """Flow with required state fields."""
+    
+    @start()
+    def begin(self):
+        return "started"
+
+
+class MixedState(BaseModel):
+    """State model with both required and optional fields."""
+    name: str  # Required
+    age: int  # Required
+    email: str = "default@example.com"  # Optional with default
+
+
+class MixedStateFlow(Flow[MixedState]):
+    """Flow with mixed required and optional state fields."""
+    
+    @start()
+    def begin(self):
+        return f"Started with {self.state.name}, {self.state.age}, {self.state.email}"
+
+
+class RequiredStateWithFlowState(FlowState):
+    """State model extending FlowState with required fields."""
+    name: str
+    age: int
+
+
+class RequiredFlowStateFlow(Flow[RequiredStateWithFlowState]):
+    """Flow with required FlowState fields."""
+    
+    @start()
+    def begin(self):
+        return "started"
+
+
+def test_flow_initialization_without_kwargs_raises_validation_error():
+    """Test that Flow initialization without kwargs raises ValidationError for required fields."""
+    with pytest.raises(ValidationError) as exc_info:
+        RequiredStateFlow()
+    
+    error_str = str(exc_info.value)
+    assert "name" in error_str
+    assert "age" in error_str
+
+
+def test_flow_initialization_with_kwargs_passes_and_sets_state():
+    """Test that Flow initialization with kwargs properly sets state values."""
+    flow = RequiredStateFlow(name="John", age=30)
+    assert flow.state.name == "John"
+    assert flow.state.age == 30
+    assert hasattr(flow.state, "id")
+    assert flow.state.id is not None
+
+
+def test_flow_initialization_with_partial_kwargs_raises_validation_error():
+    """Test that Flow initialization with only some required kwargs raises ValidationError."""
+    with pytest.raises(ValidationError) as exc_info:
+        RequiredStateFlow(name="John")
+    
+    error_str = str(exc_info.value)
+    assert "age" in error_str
+
+
+def test_flow_initialization_with_mixed_required_and_optional_fields():
+    """Test Flow initialization with both required and optional fields."""
+    flow1 = MixedStateFlow(name="Alice", age=25)
+    assert flow1.state.name == "Alice"
+    assert flow1.state.age == 25
+    assert flow1.state.email == "default@example.com"
+    
+    flow2 = MixedStateFlow(name="Bob", age=35, email="bob@example.com")
+    assert flow2.state.name == "Bob"
+    assert flow2.state.age == 35
+    assert flow2.state.email == "bob@example.com"
+
+
+def test_flow_initialization_with_flowstate_and_required_fields():
+    """Test Flow initialization with FlowState subclass having required fields."""
+    flow = RequiredFlowStateFlow(name="Charlie", age=40)
+    assert flow.state.name == "Charlie"
+    assert flow.state.age == 40
+    assert hasattr(flow.state, "id")
+    assert flow.state.id is not None
+
+
+def test_flow_execution_with_required_state():
+    """Test that Flow execution works correctly with required state fields."""
+    flow = RequiredStateFlow(name="David", age=45)
+    result = flow.kickoff()
+    assert result == "started"
+    assert flow.state.name == "David"
+    assert flow.state.age == 45
+
+
+def test_flow_with_state_modification():
+    """Test that state can be modified during flow execution."""
+    
+    class ModifiableState(BaseModel):
+        counter: int
+        name: str
+    
+    class ModifiableFlow(Flow[ModifiableState]):
+        @start()
+        def increment(self):
+            self.state.counter += 10
+            return "incremented"
+        
+        @listen(increment)
+        def check_value(self):
+            assert self.state.counter == 15
+            return "checked"
+    
+    flow = ModifiableFlow(counter=5, name="Test")
+    result = flow.kickoff()
+    assert result == "checked"
+    assert flow.state.counter == 15
+
+
+def test_flow_initialization_preserves_id_field():
+    """Test that the automatically generated id field is preserved."""
+    flow = RequiredStateFlow(name="Eve", age=28)
+    original_id = flow.state.id
+    
+    assert isinstance(original_id, str)
+    assert len(original_id) == 36  # UUID format with hyphens
+    
+    assert flow.state.id == original_id