fix(browser): address Copilot review on /browser connect

Fixes from Copilot's two passes on PR #17238:

* Validate parsed URL once: reject missing host, invalid port, and
  unsupported scheme up front so malformed inputs (e.g. http://:9222
  or http://localhost:abc) don't fall through to a generic 5031.
* Tighten _is_default_local_cdp to require a discovery-style path so
  ws://127.0.0.1:9222/devtools/browser/<id> is not collapsed to bare
  http://127.0.0.1:9222 (which would lose the path and break the
  connect).
* Move browser.manage into _LONG_HANDLERS so the up-to-10s
  launch-and-retry loop runs on the RPC pool instead of blocking the
  main dispatcher.
* try_launch_chrome_debug uses Windows-appropriate detach kwargs
  (creationflags=DETACHED_PROCESS|CREATE_NEW_PROCESS_GROUP) instead
  of POSIX-only start_new_session=True.
* manual_chrome_debug_command uses subprocess.list2cmdline on
  Windows so the printed instruction is cmd.exe-compatible.
* Mirror host/port validation in cli.py /browser connect so the
  classic CLI never persists an invalid BROWSER_CDP_URL.

ui-tui/src/__tests__/createSlashHandler.test.ts

@@ -218,6 +218,7 @@ describe('createSlashHandler', () => {
         url: 'http://127.0.0.1:9222'
       })
     )
+
     const ctx = buildCtx({ gateway: { ...buildGateway(), rpc } })
 
     expect(createSlashHandler(ctx)('/browser connect')).toBe(true)