Fix RestClient proxy path to preserve ProxyManager behavior

[Copilot]

This PR addresses the single review thread requesting a fix for proxy handling in RestClient. The previous implementation replaced ProxyManager with a host-scoped connection pool, which bypassed manager-level proxy behavior (CONNECT/proxy headers).

• What changed

  • Keep manager instances intact
    • RestClient.__init__ now stores urllib3.ProxyManager / urllib3.PoolManager directly instead of calling .connection_from_url(...).
  • Send full URLs through the manager
    • RestClient.request(...) now builds an absolute URL from base_url + path (for relative paths) and sends that URL via pool_manager.request(...).
    • Absolute URLs remain supported as-is.

• Targeted test coverage

  • Proxy behavior regression test
    • Added a focused unit test to assert proxied RestClient keeps the manager instance and issues requests with an absolute URL.

rest = RestClient(base_url="http://localhost:8181", proxy="http://proxy:3128")
rest.request(method="GET", url="/ping")
# manager.request called with: ("GET", "http://localhost:8181/ping", ...)