Fix transaction wrapping for sqlite-utils 4.0 compatibility

[bunlongheng]

Problem

sqlite-utils 4.0 changed transaction behaviour: every write (insert_all, upsert_all, execute, etc.) now checks conn.in_transaction at call-time. If that flag is False, sqlite-utils opens and immediately commits its own transaction, bypassing Datasette's outer BEGIN IMMEDIATE. A subsequent failure leaves the committed rows behind with no rollback path.

The old Datasette pattern:

with conn:
    conn.execute("BEGIN IMMEDIATE")
    result = fn(conn)

relies on Python's sqlite3.Connection context manager calling conn.commit() on exit. This is fragile: it does not guarantee that conn.in_transaction is True during fn(), and if an inner with conn: inside the write function already committed, the outer with conn: exit becomes a no-op commit on a closed transaction.

Fix

Replace both the non-threaded path (database.py:352) and the write-thread path (database.py:491) with an explicit try/except:

conn.execute("BEGIN IMMEDIATE")
try:
    result = fn(conn)
    conn.commit()
except BaseException:
    conn.rollback()
    raise

Key properties:

• conn.in_transaction is True from BEGIN IMMEDIATE until commit()/rollback(), so sqlite-utils 4.0 correctly detects the outer transaction and uses SAVEPOINT instead of auto-commit
• Python's conn.commit() / conn.rollback() are idempotent no-ops when no transaction is active, so write functions that manage their own inner with conn: (e.g. alter_table) continue to work

Testing

• All 147 test_api_write.py tests pass
• All 67 test_internals_database.py tests pass

Closes #2831