Avoid SQLite stalls from connection setup and stderr span logging - #49032

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## Why

Setting `auto_vacuum` on every connection can wait for a writer lock even when the mode is unchanged. Logging every span entry and exit can also block SQLx workers on undrained stderr while they hold a write transaction.

## What changed

- Set incremental vacuum only for empty databases or databases using full auto-vacuum, before enabling WAL. Preserve existing nonempty databases with auto-vacuum disabled.
- Emit span creation and closure events in app-server text and JSON stderr logs, preserving busy/idle timings and explicit events while omitting span entry and exit records.

## Testing

Add regression tests for opening and expanding writable pools while another connection holds a writer lock, and for transaction completion with undrained stderr in both log formats. Verify vacuum modes and retained log diagnostics.

GitOrigin-RevId: 40d4ef32c7efc49f4240b987277ba45551259335