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

---

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

• Yes (please specify the tool below)

Generated-by: OpenAI Codex following the guidelines

---

• 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.