[BUG] Transform flow: Fields added via DataScope assignment in row context don't persist to parent table schema

[lemorage]

Describe the bug
Based on the following MRE:

1. Schema Issue: The _engine_data_slice doesn't update the children table's schema to include new_field. The children table schema remains LTable(Struct(value: Int64)) instead of becoming LTable(Struct(value: Int64, new_field: Int64)).
2. Field Scope: The new_field exists temporarily within the row context (DataScope) but doesn't persist to the parent table's schema.
3. Lost on Return: When returning data, the new_field is not part of the table schema, so the decoder falls back to the default value (0) from the NewChild dataclass.

---

Relevant Context: #737

To Reproduce
Create a new test file under python/cocoindex/tests and run this MRE code:

import typing
from dataclasses import dataclass
import pytest
import cocoindex

@dataclass
class Child:
    value: int

@dataclass
class Parent:
    children: list[Child]

@dataclass
class NewChild:
    value: int
    new_field: int = 0

@dataclass
class ParentWithNewField:
    children: list[NewChild]

@pytest.fixture(scope="session", autouse=True)
def init_cocoindex() -> typing.Generator[None, None, None]:
    cocoindex.init()
    yield

@cocoindex.op.function()
def extract_value(value: int) -> int:
    return value

# Transform flow that should add new_field
@cocoindex.transform_flow()
def for_each_transform(
    data: cocoindex.DataSlice[Parent],
) -> cocoindex.DataSlice[ParentWithNewField]:
    with data["children"].row() as child:
        child["new_field"] = child["value"].transform(extract_value)
    return data

def test_for_each_transform():
    input_data = Parent(children=[Child(1), Child(2), Child(3)])
    result = for_each_transform.eval(input_data)
    print(f"Result: {result}")
    expected = ParentWithNewField(
        children=[
            NewChild(value=1, new_field=1),
            NewChild(value=2, new_field=2),
            NewChild(value=3, new_field=3),
        ]
    )
    assert result == expected, f"Expected {expected}, got {result}"

# Expected ParentWithNewField(children=
# [NewChild(value=1, new_field=1),
# NewChild(value=2, new_field=2),
# NewChild(value=3, new_field=3)]),
# got ParentWithNewField(children=
# [NewChild(value=1, new_field=0),
# NewChild(value=2, new_field=0),
# NewChild(value=3, new_field=0)])

Expected behavior

• Intended: child["new_field"] = ... should modify the table schema and persist
• Actual: The field assignment is temporary and doesn't affect the parent table schema