Fix: show all web search sources when multiple searches run in one message

[evilPaprika]

Problem

When a single assistant message triggers multiple web searches in parallel
(e.g., “Compare the current weather in Paris and London, and also tell me about recent Apple tech news”),
the UI only showed sources from one of the searches. This happened because the code used .find(...)?.sources.

What’s changed

• Replace .find with .filter(...).flatMap(...) over MessageWebSearchUpdateType.Sources updates.
• Result: the “Sources” chips now include links from all searches that contributed to the message.

Before

Only a single search’s sources appeared under the message.

After

All contributing searches’ sources appear.