depends_on_past and run lookups broken by nullable logical_date (AIP-39)

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

Airflow Core

### Apache Airflow version

3.2.0.dev0

### What happened and how to reproduce it?

In Airflow 3 the core methods for identifying previous DAG runs are strictly tied to the logical_date column causing a major regression for Asset triggered and Manual runs where this field is nullable (AIP-39) Specifically methods like get_previous_dagrun in dagrun.py (line 991) and get_last_dagrun in dag.py (line 247) return None or filter out runs when logical_date is missing which leads to depends_on_past checks being bypassed. To reproduce fail a manual run on a DAG with depends_on_past=True and trigger a second manual run; the second run will incorrectly start immediately because the scheduler fails to find and anchor on the previous failed run using the modern run_after chronostamp.

### What you think should happen instead?

The scheduler should fallback to using the run_after column for chronological lookups when logical_date is None to ensure depends_on_past still works.

### Operating System

macOS 26.4.1 (arm64)

### Deployment

None

### 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?

This problem occurs every time for non-scheduled runs (Manual/Asset-triggered) because the anchor logic strictly depends on the presence of a logical_date.

### Are you willing to submit PR?

- [x] 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)