fix(polyfill): suppress mobile-subscription billing 400 toasts

[FreezingM]

Summary / 摘要

Suppress the spurious danger toast shown on every new session when the account's ChatGPT subscription is billed through a mobile platform (Apple App Store / Google Play). The runtime's billing check returns HTTP 400 with "Payment methods for mobile subscriptions must be managed in the mobile platform." — shouldSurfaceFetchIpcError treated any 400 as a fatal IPC error, but this one is harmless and the official app silently ignores it.

修复一个问题：当 ChatGPT 订阅走移动平台（App Store / Google Play）扣费时，新建会话时的账单检查会返回 HTTP 400（"Payment methods for mobile subscriptions must be managed in the mobile platform."）。shouldSurfaceFetchIpcError 把一切 400 都当作致命 IPC 错误来弹窗，但这类账单提示对使用毫无影响，官方 App 也是静默忽略的——结果就是每次新建会话都会误弹一次故障提示。

What changed / 改动内容

• In web-shell/codex-bridge-polyfill.js, shouldSurfaceFetchIpcError now ignores 400 responses whose message matches the mobile-subscription billing pattern; all other 400s and IPC channel errors still surface unchanged.
• 在 web-shell/codex-bridge-polyfill.js 的 shouldSurfaceFetchIpcError 中，忽略消息匹配「移动订阅账单」模式的 400 响应；其余 400 与 IPC 通道错误仍照常提示，行为不变。

Verification / 验证

• New sessions no longer show the toast; non-billing 400 handling is untouched.
• 新建会话不再误弹提示；非账单类 400 的处理逻辑未改动。