Bug 1 of #55048: when the MCP connection dropped (driver crash / restart),
_lifecycle_coro exited but left _started=True, so the next list_apps/capture
passed _require_started() and then operated on a None session — hanging
forever instead of reconnecting.
- _lifecycle_coro's finally now resets _started=False on ANY exit, so a dead
session is re-enterable (idempotent no-op on the normal stop() path; atomic
bool write, safe from the bridge-loop thread without the lock stop() holds).
- call_tool() re-enters start() when the session isn't active, rebuilding it
before the call. The start_session/end_session handshake (driven by start()/
stop() themselves) is exempted so bootstrap doesn't recurse.
Tests: two cases in test_computer_use_delivery_ladder.py — finally resets
_started, and call_tool restarts a dead session exactly once. Full
computer_use suite green (233).
Refs #55048 (Bug 1). Bug 2 (expose foreground dispatch) is covered by the
delivery_mode work in #67123.
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