Fixes the four poisoned-connection classes (#81051, #77765, #84132,
#81995) with the SuspectableBackend cheap-mark/lazy-verify contract:
- mark_suspect/ensure_healthy protocol (agent/deadline.py): noticing a
poisoned state never does I/O; the NEXT caller pays once for a health
probe that clears the suspicion or forces a reconnect. A single
teardown-vs-keepalive race or auth-lock corruption can no longer park
a connection permanently — park stays reserved for genuinely
exhausted reconnect budgets.
- keepalive failure marks the connection suspect before requesting
reconnect; the next tool call probes and recycles if unhealthy.
- auth-classified permanent failures on a previously-proven session get
a suspect+reconnect path instead of an immediate park.
- fast-fail (#81995): stdio child pids are tracked at spawn and an
in-flight RPC races a child-watcher task, so a dead subprocess fails
the call immediately with a retryable timeout instead of riding out
the full 300s. Deliberate teardown/reconnect also fails in-flight
calls now instead of leaving them attached to a dying transport.
Dispatch-boundary hardening for test doubles: stubbed sessions
(MagicMock/non-awaitable call_tool, absent child-watcher) fall back to
the exact pre-change inline-await semantics, so only real transports
gain the race guard.
Salvage credit: in-flight approach from #73377 (@luijoc, wedged
transport recovery) and #48069 (@arminanton, keepalive/in-flight
interaction); both PRs' bases predate main's current park/reconnect
architecture, so this is a fresh implementation of their contracts.
Tests: tests/tools/ -k mcp = 639 passed (was 22 new failures during
development; final tree zero).
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