AST builder: preserve multiline callback Python code formatting

[YasiruRF]

Problem

Callback body extraction may lose indentation/newline fidelity for embedded Python blocks.

Proposed change

• Ensure callback body capture preserves original multiline formatting and indentation.
• Validate behavior for nested blocks and blank lines.

Acceptance criteria

• Generated callback code matches source formatting semantics.
• Add tests for multiline callback blocks with nested control flow.
• No regressions in existing callback tests.