* fix(mcp): invalidate tool prompt cache on connect/disconnect/error
get_tool_descriptions_for_prompt cached its result keyed only on
(disabled_map, len(_tools)). If a server reconnects with the same
tool count (or transitions to error state), the cache was never
busted — the agent received stale tool descriptions for the new
connection state.
Add a _generation counter incremented on every structural change
(successful connect, disconnect, connection error) and include it in
the cache key.
* test(mcp): regression test for _generation cache invalidation
