Refactor assistant source formatting out of Swing UI

[drafael]

Summary

• move citation/source Markdown transformation from ChatPanel into a package-private, Swing-independent AssistantSourceFormatter
• keep streaming state, locking, persistence, and EDT/UI updates owned by ChatPanel
• replace reflective ChatPanelTest coverage with direct deterministic unit tests, including disabled-search and answer-URL fallback cases

Design

This is intentionally a narrow extraction: one concrete stateless class, no interface, dependency injection, coordinator chain, or new lifecycle. The formatter accepts only the values it needs (String, List<CitationRef>, and the Web Search enabled flag) and remains unaware of ChatPanel, SendJob, StreamingSession, Swing, or persistence.

ChatPanel is reduced by 177 production lines while retaining orchestration and UI responsibilities.

Testing

• mvn -B -ntp -Dtest=AssistantSourceFormatterTest,ChatPanelTest test
• mvn -B -ntp verify — 2,889 tests, 0 failures/errors, 5 skipped; isolated shaded runtime verification passed