Handle empty page snapshots safely

[715494637]

Summary

• avoid selecting an undefined page when the filtered page snapshot is empty
• clear the selected page and dialog binding when all pages disappear
• add focused tests for empty page snapshots and undefined page selection

Problem

createPagesSnapshot() can currently call selectPage(this.#pages[0]) even when this.#pages is empty. That falls through to newPage.on(...) and crashes with Cannot read properties of undefined (reading 'on').

Testing

• bun run build
• node --require ./build/tests/setup.js --no-warnings=ExperimentalWarning --test-reporter spec --test build/tests/McpContext.test.js