Remove tiered_proxy_urls from ProxyConfiguration #2299

Remove tiered_proxy_urls from ProxyConfiguration, together with _ProxyTierTracker, ProxyInfo.proxy_tier and Request.last_proxy_tier. The tracker mutates the request (last_proxy_tier, forefront) from inside ProxyConfiguration, which does not fit once the proxy belongs to the session (#2298).

JS v4 dropped tiers. The replacement is named sessions with different proxies: an error_handler switches request.session_id to a session with a better proxy. Python already supports binding a request to a session through Request.session_id, so this needs an upgrade-guide section and a docs example, not new API.

JS references

• Issue: v4: remove tieredProxyUrls option in ProxyConfiguration crawlee#3597
• Implementation: refactor!: remove tieredProxyUrls support crawlee#3599

This will be a breaking change.

✍️ Drafted by Claude Code