Fix serialization of datetimes with a fixed UTC offset - #74922

The serde timezone serializer can't handle timezones that are just a fixed offset with no name. The datetime serializer saves an empty place holder and reading the value back (e.g., via Xcom pull) fails. Take the following Dag:

from datetime import datetime
from airflow.sdk import dag, task

@dag
def serde_offset_tz():
    @task
    def push_offset():
        return datetime.fromisoformat("2024-01-01T00:00:00+02:00")
    @task
    def do_something(value):
        print(repr(value), value.utcoffset())

    do_something(push_offset())
serde_offset_tz()

On main, do_something would fail with: ValueError: ZoneInfo keys must be normalized relative paths, got:

This PR adds support for timezones with a fixed, nonzero UTC offset and no name, by encoding them as a pendulum FixedTimezone. Additional tests were added to cover the new behavior. The NoNameTZ helper now returns None from utcoffset so that test_timezone_serialize_no_name still covers the fallback path (no name or fixed offset).

Was generative AI tooling used to co-author this PR?

• Yes (Claude Sonnet 5.5)

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.