fix(browser): let quit clean up a browser whose contexts were never built

[PhilMeyr]

Problem

Browser#quit guards on @client but then calls contexts.close_connections unconditionally:

def quit(wait: true)
  return unless @client

  contexts.close_connections

  @client.close
  thread = @process.stop(wait: wait)
  @client = @process = @contexts = nil
  thread
end

start can leave the browser with exactly the state that guard doesn't cover — @client set and @contexts still nil:

@client = Client.new(@process.ws_url, options)
@contexts = Contexts.new(@client)
rescue StandardError
  @process.stop
  raise
end

Contexts.new sends CDP commands (find_implicit_context, subscribe, auto_attach, discover), so a browser that dies right after the websocket handshake raises there, after @client was assigned. The rescue stops the process and re-raises without clearing the ivars.

That state is unrecoverable. restart is quit; start, so the next restart enters quit, passes return unless @client, and raises NoMethodError: undefined method 'close_connections' for nil before reaching the line that resets the ivars. Every subsequent restart fails the same way, on the same object, forever.

We hit this in production: Chrome started, aborted a moment later, and the long-lived browser singleton in our PDF worker was wedged until the process was restarted by hand. Downstream code that reasonably checks browser.client before restarting sees a non-nil client and never even attempts recovery.

Fix

Safe-navigate the contexts call. quit then does the right thing for a half-started browser: @client.close releases the websocket and its thread, @process.stop reaps the process, and the ivars are cleared, so the following start runs on a clean object.

This is deliberately the smaller of the two possible fixes. Clearing @client/@contexts in start's rescue instead would leave the client unclosed — letting quit handle it keeps the existing cleanup path responsible for cleanup.

The adjacent page&.close / context&.dispose in create_page guard the same way, for the same reason (#582).

Regression spec

Added to #restart: stub Contexts.new to raise, let one restart fail, un-stub, and assert the browser restarts. Without the fix it fails with

expected no Exception, got #<NoMethodError: undefined method 'close_connections' for nil>
  ./lib/ferrum/browser.rb:252:in 'Ferrum::Browser#quit'
  ./lib/ferrum/browser.rb:234:in 'Ferrum::Browser#restart'

bundle exec rspec spec/browser_spec.rb — 52 examples, 0 failures, 3 pending. Rubocop clean.

[route]

I think we need to use attr_reader vs ivar and browser_spec is for systems spec, not unit tests