fix: fixes pydantic parser to support nested objects.

This fails before the PR and succeeds afterward:
```python
from pydantic import BaseModel
from typing import List, Optional

from crewai.utilities.pydantic_schema_parser import PydanticSchemaParser

class InnerModel(BaseModel):
    inner_field: int

class TestModel(BaseModel):
    simple_field: str
    list_field: List[int]
    optional_field: Optional[str]
    nested_model: InnerModel

print(PydanticSchemaParser(model=InnerModel).get_schema())   # works
print(PydanticSchemaParser(model=TestModel).get_schema())    # fails
```

**Note.** Because the `main` branch currently doesn't support nested
schemas and the original code against which I made this PR months ago
drifted, I made a judgement call on how to format the nested
structurees. I chose the following, inferred from the current code:

````

>>> print(PydanticSchemaParser(model=InnerModel).get_schema())
{
    inner_field: int
}

>>> print(PydanticSchemaParser(model=TestModel).get_schema())
{
    simple_field: str,
    list_field: List[int],
    optional_field: Optional[str],
    nested_model: InnerModel
    {
        inner_field: int
    }
}

tests/utilities/test_pydantic_schema_parser.py

@@ -0,0 +1,73 @@
+import unittest
+from pydantic import BaseModel
+from typing import List, Optional
+
+from crewai.utilities.pydantic_schema_parser import PydanticSchemaParser
+
+
+# Define test models
+class InnerModel(BaseModel):
+    inner_field: int
+
+
+class OuterModel(BaseModel):
+    simple_field: str
+    list_field: List[int]
+    optional_field: Optional[str]
+    nested_model: InnerModel
+
+
+# Test cases
+class TestPydanticSchemaParser(unittest.TestCase):
+    def test_simple_model(self):
+        class SimpleModel(BaseModel):
+            field1: int
+            field2: str
+
+        parser = PydanticSchemaParser(model=SimpleModel)
+        expected_schema = """
+{
+    field1: int,
+    field2: str
+}""".strip()
+        self.assertEqual(parser.get_schema(), expected_schema)
+
+    def test_model_with_list(self):
+        class ListModel(BaseModel):
+            field1: List[int]
+
+        parser = PydanticSchemaParser(model=ListModel)
+        expected_schema = """
+{
+    field1: List[int]
+}""".strip()
+        self.assertEqual(parser.get_schema(), expected_schema)
+
+    def test_model_with_optional(self):
+        class OptionalModel(BaseModel):
+            field1: Optional[int]
+
+        parser = PydanticSchemaParser(model=OptionalModel)
+        expected_schema = """
+{
+    field1: Optional[int]
+}""".strip()
+        self.assertEqual(parser.get_schema(), expected_schema)
+
+    def test_nested_model(self):
+        parser = PydanticSchemaParser(model=OuterModel)
+        expected_schema = """
+{
+    simple_field: str,
+    list_field: List[int],
+    optional_field: Optional[str],
+    nested_model: InnerModel
+    {
+        inner_field: int
+    }
+}""".strip()
+        self.assertEqual(parser.get_schema(), expected_schema)
+
+
+if __name__ == "__main__":
+    unittest.main()