Pre-populate a suggested Partition Key when manually triggering temporal partitioned DAGs

### Description

When manually triggering a DAG using a temporal partitioned timetable, the trigger form leaves partition_key blank.

Submitted the form without a key created an unpartitioned DagRun. Although the run may execute successfully, its asset events cannot satisfy partition-aware downstream DAGs. This is an easy operational footgun, particularly for daily asset-driven pipelines.

### Use case/motivation

Operators frequently need to manually trigger or rerun daily partitioned DAGs. Currently, they must enter the partition key manually on every trigger.

It would be useful if the form could derive a suggested key, perhaps using some custom function.

### Related issues

_No response_

### Are you willing to submit a PR?

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

### Code of Conduct

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