fix(channels): safely parse non-string user message content

[arpan7sarkar]

Problem

In toVisibleChatItems (app/src/components/channels/chat-messages.ts), non-string message.content was assumed to always be an array. If message.content was undefined, null, or an unexpected object format, calling .filter() threw an
unhandled TypeError that crashed the React chat transcript.

Solution

• Added an Array.isArray(message.content) check before filtering array parts.
• Added an isTextPart type guard to safely extract text from valid { type: "text", text: string } items and ignore null/undefined/malformed elements.
• Added comprehensive unit tests in app/tests/chat-messages.test.ts.

Verification

• bun test app/tests/chat-messages.test.ts (7/7 passed)
• bun run typecheck (0 errors)
• bunx biome check (0 errors)

closes #44