[Bug] ArrowFlight SQL result automatically converts timezone naive date to UTC (which is wrong)

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

When using arrow to execute sql:
SELECT CAST('2026-07-02 01:36:22.069504' AS DATETIME(6)) AS ts

### What's Wrong?

The result is automatically converted to UTC.
2026-07-02 01:36:22.069504+00:00

### What You Expected?

The result sould be
2026-07-02 01:36:22.069504

This is the wrong assumption. the original timestamp might be local timezone timestamp.

### How to Reproduce?

Execute
SELECT CAST('2026-07-02 01:36:22.069504' AS DATETIME(6)) AS ts

in python:
import adbc_driver_flightsql.dbapi as flightsql
from adbc_driver_manager import DatabaseOptions
from q_secrets import get_connection_details

details = get_connection_details("doris", flightsql=True)
connection = flightsql.connect(
uri=f"{details['driver']}://{details['hostname']}:{details['port']}",
db_kwargs={
DatabaseOptions.USERNAME.value: details["username"],
DatabaseOptions.PASSWORD.value: details["pwd"],
},
)

with connection.cursor() as cursor:
cursor.execute("SELECT CAST('2026-07-02 01:36:22.069504' AS DATETIME(6)) AS ts")
table = cursor.fetch_arrow_table()

field = table.schema.field("ts")
print("Arrow field:", field)
print("Field metadata:", field.metadata)
print("Value:", table["ts"].to_pylist())

connection.close()

### Anything Else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start by running the provided Python Arrow Flight SQL reproduction with the DATETIME(6) query, then inspect the Arrow field metadata and value shown in the issue. Trace where the timezone-naive result is converted to UTC; done means the value remains 2026-07-02 01:36:22.069504 without a UTC offset.

Written by the indexing model from the issue text.