feat(browser_eval): add executablePath support for custom browser locations

[pwarnock]

Summary

Adds executablePath parameter to browser_eval tool for specifying custom browser binary locations.

Motivation

Users with Homebrew-installed Playwright browsers cannot use browser automation because browsers are installed at non-standard locations (~/Library/Caches/ms-playwright/). This parameter passes through to playwright-mcp's --executable-path flag.

Use Case

browser_eval({
  action: "start",
  browser: "chrome",
  executablePath: "~/Library/Caches/ms-playwright/chromium-1200/.../Google Chrome for Testing"
})

Changes

• Added executablePath to startBrowserEvalMCP() options interface
• Added executablePath to browser_eval tool input schema
• Updated tool description with usage example
• Added unit tests for the new parameter

Testing

• Unit tests pass (npm run test)
• Build succeeds (npm run build)
• Manual verification with custom executable path