Add assert for tables.callback before calling

Click's callback attribute is typed as Optional[Callable], so add
assert to satisfy type checker that it's not None.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sqlite_utils/cli.py

@@ -293,6 +293,7 @@ def views(
     \b
         sqlite-utils views trees.db
     """
+    assert tables.callback is not None
     tables.callback(
         path=path,
         fts4=False,