Avoid false TriggerDagRunOperator conflicts after ambiguous trigger retry

[hkc-8010]

Avoid retrying the non-idempotent trigger Dag run POST through the generic execution API retry layer.

When the trigger POST raises an ambiguous transport-level httpx.RequestError, the client now probes the requested Dag run ID. If the run exists and reset_dag_run=False, it treats the trigger as successful. If the run is missing, it re-raises the original request error.

This preserves direct 409 Conflict behavior, including DAGRUN_ALREADY_EXISTS and the existing reset_dag_run=True clear path.

closes: #66905

Tests:

• uv run --project task-sdk ruff format task-sdk/src/airflow/sdk/api/client.py task-sdk/tests/task_sdk/api/test_client.py
• uv run --project task-sdk ruff check --fix task-sdk/src/airflow/sdk/api/client.py task-sdk/tests/task_sdk/api/test_client.py
• uv run --project task-sdk pytest task-sdk/tests/task_sdk/api/test_client.py::TestDagRunOperations -q
• prek run --files task-sdk/src/airflow/sdk/api/client.py task-sdk/tests/task_sdk/api/test_client.py
• breeze testing task-sdk-tests --python 3.10 task-sdk/tests/task_sdk/api/test_client.py::TestDagRunOperations
• breeze testing task-sdk-tests --python 3.10
• breeze ci selective-check --commit-ref HEAD --default-branch main --github-repository apache/airflow

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