Leave generated columns out of Export to SQL - #4198

Fixes #2978.

Export → Database to SQL file writes every column of a table into its INSERT statements, generated columns included. SQLite doesn't allow inserting into a generated column, so the exported file can't be imported again:

CREATE TABLE orders(id INTEGER PRIMARY KEY, qty INTEGER, price REAL,
                    total REAL GENERATED ALWAYS AS (qty*price) STORED,
                    label TEXT GENERATED ALWAYS AS ('order ' || id) VIRTUAL,
                    note TEXT);

• Default export: INSERT INTO "orders" VALUES (1,2,9.5,19.0,'order 1','O''Brien'); fails with table orders has 4 columns but 6 values were supplied.
• With "Keep column names in INSERT INTO": fails with cannot INSERT into generated column "total".

On #2978 it was suggested that the export should keep all columns. The sqlite3 shell's .dump leaves generated columns out (INSERT INTO t VALUES(1);), because SQLite recomputes them on import. This change makes DB4S behave the same way, so its SQL export can be re-imported.

Change

In DBBrowserDB::dump(), tables that have at least one generated column select, and name in the INSERT statements, only their other columns. Tables without generated columns keep the existing SELECT *, so their export is byte-for-byte unchanged. The same file already skips generated columns this way in emptyInsertStmt() and when copying data during alterTable().

Test plan

Built on macOS (Apple Silicon, Qt 5.15.16 from the project tap) with -DENABLE_TESTING=ON. ctest passes (4/4). None of those tests cover the export.

Exported the orders table above, which has a stored and a virtual generated column, a quoted value and a NULL, together with a plain table. I used each export option and re-imported every file into a fresh database with sqlite3:

The plain table exports as before in all three.

CSV import into an existing table with generated columns has a similar problem (ImportCsvDialog.cpp). I've kept this PR to the SQL export and can send that as a separate PR if it's wanted.