fix: skip frozen page targets when page enumeration times out

[barretlee]

Summary

This PR makes page enumeration resilient to frozen/discarded background tabs.

When browser.pages() fails because Puppeteer times out while initializing a page session (for example on Network.enable), MCP now falls back to enumerating page targets one by one and skips only the targets that hit the same recoverable timeout.

Motivation

Closes #1230.

In a long-lived Chrome profile, some background tabs can become frozen or discarded by Chrome memory-saving behavior. Before this change, a single bad target could make page enumeration fail for the entire MCP session.

Changes

• Keep the normal browser.pages() path for healthy browsers
• Fall back to per-target enumeration when page initialization fails with recoverable page-session timeouts
• Skip only the failing targets and keep healthy pages available
• Add a regression test covering the fallback path

Verification

• npm run typecheck
• npm test -- tests/McpContext.test.ts
• Manual repro against a real Chrome session with frozen background tabs:
  • raw CDP Network.enable still timed out for affected targets
  • patched McpContext.from() succeeded and skipped the frozen targets

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