fix/task-group-branching: Allowing for relative paths to be branched to in a task group

[jroachgolf84]

Description

This PR allows for the proper resolution of relative paths when branching within a Task Group.

For example, if a Task Group tg has a branching task and a task a and b, returning "a" from the branching task will result in a executing and b getting skipped. This is shown in the DAG/screenshot below.

However, if there is both a top-level a and an a in the Task Group, a will be skipped in the Task Group.

related: #65751

Testing

These changes were tested in two different ways; via unit tests, as well as E2E testing. To execute the unit tests added as part of this PR, the commands below can be run:

breeze testing providers-tests providers/standard/tests/unit/standard/operators/test_branch_operator.py

breeze testing task-sdk-tests providers/standard/tests/unit/standard/operators/test_branch_operator.py

To test these changes E2E, the DAG below was used. To ensure the changes were correct, the behavior expected would be:

• Task a in tg executes
• Task tg.c executes
• Task e inside of tg executes

from airflow.sdk import DAG, task, task_group
from datetime import datetime

with DAG(
    dag_id="fix_task_group_branching",
    start_date=datetime(2026, 1, 1),
    schedule="@once"
) as dag:

    @task_group()
    def tg():

        @task.branch
        def choose_tg_relative():
            return "a"

        @task
        def a():
            # Should execute
            return None

        @task
        def b():
            # Should be skipped
            return None

        choose_tg_relative() >> [a(), b()]

        @task.branch
        def choose_tg_absolute():
            return "tg.c"

        @task
        def c():
            return None

        @task
        def d():
            return None

        choose_tg_absolute() >> [c(), d()]

    tg()