[Airflow 3 / Task SDK] Heartbeat 404 "Task Instance not found" after ~12h run; supervisor SERVER_TERMINATED (SIGKILL); retry sees 409 while TI still running

### Under which category would you file this issue?

Task SDK

### Apache Airflow version

airflow 3.0.6

### What happened and how to reproduce it?

During a long-running task (wall time on the order of ~12 hours in our case), the task process was terminated by the supervisor after the execution API stopped accepting heartbeats:
- PUT /execution/task-instances//heartbeat returned 404 with Task Instance not found / reason: not_found.
- Supervisor logged that the server indicated the task should not keep running, then Task killed! with process SIGKILL (exit_code=-9), final_state=SERVER_TERMINATED.
- Supervisor also logged duration ~ 43214 s (~12 h) for that workload session.

In the same log window we also saw PATCH .../task-instances//run return 409 Conflict with a message along the lines of cannot start task instance — invalid state, previous_state=running (exact try_number in our export was 2; we treat that as illustrative, not the core claim—we care about 404 + forced termination after a long running execute, regardless of attempt index).

DAG/operator architecture: The workload is a PythonOperator subclass whose execute() submits work to Snowflake and then blocks inside the same task on an imperative wait() (implemented on a BaseSensorOperator subclass: time.sleep + polling). There is no separate sensor task, so one TI can stay running for many hours.

What I need help with:

1. Under what conditions does the Task Execution API return 404 on heartbeat for a TI that was recently updating heartbeats successfully?
2. Whether long running tasks on Celery + Airflow 3 are expected to hit this, and how it relates to timeouts, TI lifecycle, or DAG run / span updates (we also see scheduler INFO such as Found span_status 'should_end', while updating state for dag_run ... on the same run).
3. Recommended patterns for multi-hour waits (deferrable operators, separate sensor task, etc.) under the Task SDK.

Representative log excerpts:
`PUT /execution/task-instances//heartbeat HTTP/1.1" 404 Not Found
Task Instance not found ti_id=
Server indicated the task shouldn't be running anymore ... reason='not_found' message='Task Instance not found'
Task killed!
Task finished [supervisor] duration=43214.24... exit_code=-9 final_state=SERVER_TERMINATED`

### What you think should happen instead?

Heartbeat should keep returning 2xx until the TI is terminal or explicitly cleared; worker should not get 404 while still the active runner.

### Operating System

_No response_

### Deployment

MWAA, Celery

### Apache Airflow Provider(s)

_No response_

### Versions of Apache Airflow Providers

_No response_

### Official Helm Chart version

Not Applicable

### Kubernetes Version

_No response_

### Helm Chart configuration

_No response_

### Docker Image customizations

_No response_

### Anything else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)