Fix partition key display and input handling

[Lee-W]

Why

The trigger/materialize forms and run views mishandled partition keys — non-partitioned Dag runs showed a dangling "Partition key:" label, an emptied key was sent to the API and rejected with a 400, and the field appeared even where partitioning doesn't apply.

What

• Hide the partition key line/stat when partition_key is null (run header, asset event view)
• Normalize an empty or cleared partition key (""/undefined) to null before hitting the trigger, materialize, and asset-event APIs — no more 400 on a blank field
• Show the trigger form's partition key field only for partitioned Dags, with a helper hint
• Swap the manual asset-event partition key from a JsonEditor to a plain text Input — it's a string, not JSON
• Default the runs table's partition_key column to visible for a partitioned Dag's runs tab, hidden elsewhere
• Unify the label — cron/runtime-partitioned runs now say "Partition key" instead of "Mapped Partition key"

non partitioned

partitioned

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