Before this fix, _chromium_installed() only searched Playwright-style
chromium-* / chromium_headless_shell-* directories, which meant users
with system Chrome or AGENT_BROWSER_EXECUTABLE_PATH configured still
had all browser_* tools gated.
Now checks three sources in priority order:
1. AGENT_BROWSER_EXECUTABLE_PATH env var (if set and points to a real binary)
2. System Chrome/Chromium via shutil.which() (google-chrome, chromium-browser, chrome)
3. Playwright browser cache (existing logic, kept as fallback)
Closes#19294
