[Paul Lizer (paullizer)]

Summary

• Fixes Merge prior grounded sources into follow-up mixed-source tasks #1204.
• Detects explicit follow-up references to prior grounded sources such as that XML file, same template, and previous spreadsheet.
• Revalidates prior grounded source refs and merges them with current selected/retrieved source IDs before standard and streaming mixed-source resolution.
• Adds metadata, fix docs, release notes, and focused regression coverage for the prior-source merge behavior.

Validation

• python functional_tests\test_chat_history_grounded_follow_up_fix.py
• python functional_tests\test_mixed_source_conversation_continuity.py
• python -m py_compile application\single_app\route_backend_chats.py application\single_app\config.py functional_tests\test_chat_history_grounded_follow_up_fix.py
• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --check