Fix Grid view failing to load large Dags on MySQL

[WonYong-Jang]

Grid view showed no tasks for large Dags when the metadata backend is MySQL.
The API server logged:

ERROR 1038: Out of sort memory, consider increasing server sort buffer size

Root cause

The grid endpoint used _get_latest_serdag to load the latest serialized Dag.
That query did select(SerializedDagModel) ... ORDER BY ... LIMIT 1, which loads the whole row - including the big data / data_compressed blob columns.

On MySQL, ORDER BY uses filesort, and filesort copies each candidate row (the blob too) into the sort buffer. The sort buffer has a fixed size (sort_buffer_size, 256 KB by default). When a single serialized Dag is bigger than that, the sort fails with "Out of sort memory", so the whole request fails and the Grid view renders nothing.

PostgreSQL is not affected because it does not copy large values into the sort buffer.

Fix

Use the existing SerializedDagModel.latest_item_select_object helper.
On MySQL it first finds the latest row's id (ORDER BY on id only, no blob), and then loads that single row by primary key. The blob is never put into the sort buffer, so the error is gone.

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