## Why
An MCP connection or tool catalog can change after a model step captures its
available tools. Calls from that step must not be rerouted to a replacement
client or run against a catalog revision the model did not see.
## What changed
- Add `McpBinding` to capture the ready clients, visible tools, resources, and
server metadata for one sampling request.
- Prepare calls against the captured client and reject them if the tool catalog
changes before call preparation and execution finish.
- Keep resource access and cache identity tied to the binding's exact connection
set, and omit servers that do not have a ready client.
- Preserve each client's unfiltered tool catalog so bindings can apply the
current filter consistently.
## Testing
Add coverage for connection replacement, stale catalog rejection, serialized
call preparation and refresh, ready-client capture, and binding-scoped resource
identity.
GitOrigin-RevId: a25da4d2c7f5ed06c41caa0c6d15c464c1698314
65f8bf6853