fix: track active browser tabs

[naliazheli]

Summary

• make newly opened Playwright pages the active target for subsequent browser tools
• add browser_list_tabs and browser_switch_tab for explicit multi-tab control
• fall back to the most recent open tab when the active tab closes
• clarify that snapshot, text, and screenshot tools operate on the active tab

Root cause

The browser controller stored only the first Page created for its context. Popups opened successfully in Chromium, but screenshots, snapshots, clicks, waits, and text reads continued using that original page. The tool schema also had no way for the model to list or select tabs.

User impact

Browser agents now follow popup/new-tab navigation automatically and can explicitly inspect or switch tabs when a workflow spans multiple pages.

Validation

• 1176 passed, 1 skipped
• real headless Chrome smoke test: popup becomes active and closing it restores the prior tab
• git diff --check