perf(export): hand large copies to the isolate in chunks (#1537)

[ZhuchkaTriplesix]

Closes #1537. The dead resultGridAsCsvAsync was already removed in #1606; this is the other half, the hand-off stall.

What stalls

DataExportService.formatAsync ran one Isolate.run whose closure captured all rows. The captured object graph is copied on the calling (UI) isolate before the worker starts, so copying 50,000 × 20 cells to the clipboard stopped the UI for ~20–30 ms.

Change

• Up to handOffCells (20,000 cells) nothing changes: one isolate, the whole table.
• Above that, the rows go over in chunks of about 20,000 cells. Each chunk is formatted on its own isolate, the event loop runs between chunks, and the parts are joined.
• formatChunk makes each part so that the join is exactly what formatCsv / formatJson / formatMarkdownTable / formatSqlInsertDump give for all rows:
  • CSV and Markdown: the header goes in the first chunk only;
  • JSON: [ in the first chunk, ] in the last, and the items are cut out of the indented array;
  • SQL dump: rows are independent.
• The isolate calls are separate functions so that each closure captures only its own arguments. A closure created inside formatAsync would carry the whole rows along; in my first measurement that made chunking slower than no chunking.
• formatMarkdownTable is split into header and rows helpers. Its output is unchanged.

Measured

Standalone Dart AOT, the issue's shape: 50,000 × 20 CSV, Timer.periodic(1 ms) on the calling isolate, longest gap during the call, 3 runs.

	longest UI gap	total
one closure (before)	20–28 ms	77–96 ms
1,000-row chunks	2–4 ms	50–58 ms
2,000–10,000-row chunks	3–16 ms	55–74 ms

20,000 cells per chunk is the 1,000-row case at 20 columns. The clipboard call itself (Clipboard.setData with one big string) is not part of this.

Tests (test/shared/data_export_service_test.dart)

For every format, chunks of 1, 3, 7, 22 and 23 rows joined equal the whole document. The rows include quotes, commas, newlines, |, NULL and short rows. formatAsync on ~40,000 cells (3 chunks) equals the whole document.

Not run with flutter test locally; CI decides.