[route]

Context#attach_target on a target that already had a session opened another one. Chrome reported it through Target.attachedToTarget, so Ferrum treated it like an auto-attach: it re-armed auto-attach on the new session, which attached every worker again, registered the extensions again, so they ran once more per document, and overwrote target.session_id while the cached Page/Worker stayed on the first session.

Fix

• attach_target returns the target as is when it already has a session, like Puppeteer's target.page()/target.worker() reuse the session they have.
• It returns the Target instead of true, so browser.attach_target(id).worker works. A target is truthy, so callers checking the result keep working.
• Ferrum never handled Target.detachedFromTarget, so a detached target kept a dead session_id and the early return would have skipped the attach. The session id is now cleared on Target.detachedFromTarget, and right away when Ferrum lets go of a service worker, so attach_target right after the worker shows up doesn't pick the session that is about to be detached.

Test

• spec/context_spec.rb: attaching twice to a page with an extension returns the target, keeps its session and the extension runs once after a reload. Without the fix the session changes and the extension runs three times.
• spec/worker_spec.rb: a detached service worker target has no session id.