Lease proxies for write and isolated callbacks; close() never frees a busy connection

Callbacks passed to execute_write_fn() - and the internal callbacks of
execute_write(), execute_write_many() and execute_write_script(), plus
write_wrapper hooks - and execute_isolated_fn() now receive a
LeasedConnection that raises ConnectionLeaseError once the callback has
returned, in threaded and non-threaded mode. "with conn as c" yields the
proxy, not the raw connection. Write callbacks may still return a cursor
(lambda conn: conn.execute(...) is common, and the write_many/script
methods return theirs).

The threaded execute_isolated_fn() path for immutable databases now runs
as a tracked executor future, so Database.close() waits for it instead
of closing its connection mid-query (the segfault in the review, P03).
close() also leaves alone the connections of a write thread that is
still running after the 10s join; that thread closes them itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwkLo2rJWidpkDVT3HkvNv

tests/test_internals_database.py

@@ -1319,9 +1319,14 @@ async def test_close_releases_memory_connections(num_sql_threads, named):
     read_connection = await db.execute_fn(
         lambda conn: conn.execute("select 1").connection
     )
-    write_connection = await db.execute_write_fn(lambda conn: conn)
+    leased_write = await db.execute_write_fn(lambda conn: conn)
+    write_connection = await db.execute_write_fn(
+        lambda conn: conn.execute("select 1").connection
+    )
     with pytest.raises(ConnectionLeaseError):
         leased.execute("select 1")
+    with pytest.raises(ConnectionLeaseError):
+        leased_write.execute("select 1")
     ds.close()
     for conn in (read_connection, write_connection):
         with pytest.raises(sqlite3.ProgrammingError, match="closed"):