Fix rescheduled sensors hanging before poke

[hkc-8010]

Rescheduled tasks currently fetch their first task reschedule start date from the supervisor when RuntimeTaskInstance.get_first_reschedule_date() is called. For sensors, this can happen before poke() starts, which means a worker can block on the supervisor/API round trip before emitting the sensor's usual poke log line.

This PR includes the first task reschedule start date in the task instance run context returned by the Execution API. The Task SDK uses that value directly when present, while keeping the existing supervisor request as a compatibility fallback for older API responses.

closes: #68010

Tests:

• breeze testing task-sdk-tests --python 3.10 -- task-sdk/tests/task_sdk/execution_time/test_task_runner.py -k get_first_reschedule_date -q
• breeze testing core-tests --python 3.10 --db-reset -- airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py airflow-core/tests/unit/api_fastapi/execution_api/versions/v2026_06_30/test_task_instances.py -q
• breeze testing core-tests --backend postgres --python 3.10 --db-reset -- airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py airflow-core/tests/unit/api_fastapi/execution_api/versions/v2026_06_30/test_task_instances.py -q
• breeze testing task-sdk-tests --python 3.10
• prek run --files <changed-files>
• prek run --files <changed-files> --stage manual

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[potiuk]

@hkc-8010 A few things need addressing before review — see our Pull Request quality criteria.

• ❌ Merge conflicts with main. See docs.

No rush.

Note: This comment was drafted by an AI-assisted triage tool and may contain mistakes. Once you have addressed the points above, an Apache Airflow maintainer — a real person — will take the next look at your PR. We use this two-stage triage process so that our maintainers' limited time is spent where it matters most: the conversation with you.

[hkc-8010]

@potiuk thanks for the heads up. I rebased the branch on current main, resolved the conflicts, and force-pushed the updated commit (f3bd4aae0da6ad23a2b06500b6c9354dec140a77).

The local pre-push hooks are passing now, including the supervisor schema snapshot/version checks that caught one missing generated snapshot update during the rebase. GitHub checks have restarted on the updated commit.

Could you please take another look when you get a chance?