* Mirror front protocol era onto proxy backend connection
A proxy created from a non-Client target now negotiates, on its backend,
whatever era its front client negotiated, instead of pinning one era.
Explicit create_proxy(mode=...) still overrides. Guards the eager backend
initialize() so an explicit modern pin behind a handshake front no longer
crashes.
* Carry the mirrored proxy era into multi-server config backends
A multi-server MCPConfig target mounts one proxy per configured server on a
composite router, so setting the era on the outer client stopped at the router
and every real backend stayed on its default era. TransportOptions.backend_mode
carries it down, resolved per request alongside the outer mirroring.
The router is also sealed under a policy held on the transport rather than a
fresh per-router ephemeral key, so a guard tool's request_state survives the
router being rebuilt between rounds.
* 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.
