chore: tighten TypeScript guards, silence stray rejections

[haotianliangye]

A grab bag of small changes that keep the dev experience clean under the strict TypeScript / Next 16 dev overlay:

TypeScript type guards on AssistantMessage.content:

• app/api/models-config/test/route.ts
• components/ChatWindow.tsx
• lib/session-title.ts

@earendil-works/pi-ai's AssistantMessage.content is typed as AssistantContentBlock[], which is a discriminated union. The naive .filter((b) => b.type === "text") produced a (AssistantContentBlock | { type: 'text'; text: string })[] that the compiler still treated as the wide type. Switching to .filter((b): b is TextContent => ...) narrows the result so the downstream .map((b) => b.text) typechecks without an as TextContent cast.

hooks/useTheme.ts:

• document.startViewTransition is not yet in the default lib types on every platform we ship to. Replace the unguarded access with a type guard, and bail to the synchronous apply() path when the API is missing so the theme still flips on browsers without View Transitions.
• No runtime behavior change for browsers that have the API.

hooks/useAgentSession.ts:

• loadModels already swallows its own errors (network failures, JSON parse failures, AbortError on cleanup). Add a defensive .catch(() => {}) so anything that ever bubbles up is silenced — Next 16's dev overlay still reports the AbortError source line even when caught, which previously surfaced as an unhandledRejection noise.

components/ChatWindow.tsx:

• Same AbortError-catching comment near the new-session update check.
• Same TextContent guard on the user-message text extraction path.