fix(stream-transform): preserve objects with a non-callable then field

[20ns]

Returning { then: "next" } from a handler currently fails with TypeError: result.then is not a function. This can happen when transforming CSV records with a column named then.

Check that then is a function before calling it. The existing object-return test now covers a string-valued then field.

The regression fails before the fix and passes afterward. Checked on Node.js 22.22.0:

• All 81 stream-transform tests and TypeScript checks pass.
• All 14 workspace checks pass.
• The package build, changed-file lint and formatting checks pass.
• Callback, stream and sync checks preserve ordinary objects. Native promises and callable thenables still work.