Fix AH msbench and extra untitled session creation (#316516)

Workbench owns provisional agent-host session config

Maintain a workbench-side cache of session config on each provisional
entry, seeded by _getInitialConfig() at create time and updated
synchronously by the picker via a new applyConfigChange API. tryRebind
copies straight from this cache instead of reading the agent's
state.config.values, which lags behind by a server roundtrip and was
dropping workbench-seeded defaults (autoApprove, isolation) on the new
backend session. Also tombstones rebound URIs so a stale onDidChange
or post-rebind getOrCreate cannot resurrect the provisional and orphan
a session on the agent.

(Written by Copilot)