* fix: cache component lists in ProxyProvider to avoid redundant backend connections
Every call_tool through a proxy was triggering _list_tools() to resolve
the tool by name, opening a full MCP session just for the lookup, then
opening a second session for the actual execution. This caches component
lists on the ProxyProvider with a configurable TTL (default 300s),
cutting backend handshakes in half for repeated calls.
* docs: document component caching and session reuse for proxy providers
* fix: add sleep in cache TTL test for Windows clock resolution
* docs: clarify cache scope and dynamic backend guidance
Rename Visibility to Enabled, collapse VisibilityRule into the transform,
and move enabled filtering from Provider to Server level so server-level
transforms can override provider-level disables.
