Build Retrieval Context with Citations

Implement format_context(chunks) where each chunk is a dict {"text": str, "source_id": str}. - Return a single formatted string. - Label each chunk with a citation marker like [1], [2], etc. in order of appearance. - Place the citation marker immediately before the chunk's text. - Separate chunks with a double newline (\n\n). - The source_id itself should not appear in the final output — only the citation number.