fix(mobile): bound AI content search tool hangs

[codedogQBY]

Analysis

• [Bug] 安卓端ai对话时一旦开始搜索内容就会卡住 #410 reports Android chat becoming unusable after the AI starts a content search tool: the UI remains in tool-calling state and the model never receives the next tool result.
• The mobile fallback content path can leave a Promise pending forever if the hidden extractor WebView never replies. It also reads the whole original book into JS memory before extraction, which is risky on Android for larger books.
• The agent also had no per-tool timeout, so any stalled tool could keep the streaming turn open indefinitely.

Changes

• Add a core fallback content provider timeout so original-file fallback extraction cannot leave a tool call pending forever.
• Add a per-tool timeout in the reading agent and return a structured { error } result when a tool runs too long.
• Add timeout/unmount cleanup for Expo ExtractorWebView extraction requests.
• Guard mobile original-file fallback search against remote/missing files and files larger than 12 MB before reading bytes into JS memory.
• Add focused tests for stalled fallback providers and stalled agent tools.

Verification

• pnpm exec biome check packages/core/src/ai/fallback-content-service.ts packages/core/src/ai/__tests__/fallback-content-service.test.ts packages/core/src/ai/agents/reading-agent.ts packages/core/src/ai/__tests__/reading-agent-tools.test.ts packages/app-expo/src/components/rag/ExtractorWebView.tsx packages/app-expo/src/screens/LibraryScreen.tsx
  • Passes with existing noExplicitAny warnings in packages/core/src/ai/agents/reading-agent.ts.
• pnpm --filter @readany/core test -- fallback-content-service reading-agent-tools
• pnpm --filter @readany/core exec tsc --noEmit
• pnpm --filter @readany/app-expo exec tsc --noEmit
• git diff --check

Fixes #410