feature: Include sources used by AI agent in final JSON report

[LuizFNJ]

Description
Enhanced the fact-checking report generation to include all sources used by the AI agent during research. This includes URLs from web searches and, potentially, public gazettes. The sources are now captured, deduplicated, and structured in the final JSON report under a dedicated "sources" field, improving transparency and verifiability of the reports.

Related Ticket #8

Type of change
New feature (non-breaking change which adds functionality)

Testing

• Verify that all URLs collected during AI research appear in the "sources" field of the final JSON report.
• Confirm that duplicate URLs are removed and each source includes a title, URL, and type.
• Test with claims that use web search only, gazettes only, and a combination of both.
• Ensure the inclusion of sources does not affect the content or formatting of the "messages" or other fields in the report.