fix(mcp): survive transient WS transport drops for Box stdio MCP servers (#2303)

* fix(mcp): survive transient WS transport drops for Box stdio MCP servers

A Box-backed stdio MCP server (e.g. pab1it0/prometheus) would periodically
error on the frontend with Box managed process exited unexpectedly /
Failed after 4 attempts once the session had been alive for a while.

Root cause: the managed MCP process lives in the Box runtime and SURVIVES a
WebSocket transport drop, but _lifecycle_loop treated any monitor completion
as a fatal process death. It then ran the finally-block cleanup — which STOPS
the still-healthy managed process — and did a full 4-attempt exponential
backoff rebuild. Under an occasionally-stalled single-worker event loop the
mcp websocket client misses a ping/pong, the transport drops, and this
self-inflicted teardown loop is what the user sees.

Fixes:
- _lifecycle_loop: when the health monitor completes, re-check the real
  managed-process state. If the process is still running, the transport
  merely dropped: raise an internal _TransportReconnect signal instead of
  Box managed process exited unexpectedly.
- _lifecycle_loop_with_retry: handle _TransportReconnect as a free, uncounted
  reconnect (does not consume the fatal retry budget), so a long-lived session
  survives arbitrarily many transient drops.
- finally-block: gate managed-process teardown on a _preserve_managed_process
  flag so a transport-only reconnect closes just the WS, not the process.
- BoxStdioSessionRuntime.initialize: reuse an already-running managed process
  instead of stopping+rebuilding it (which also re-ran the slow dependency
  bootstrap); only (re)start when none is running. Adds
  _managed_process_is_running() helper.

Pairs with langbot-plugin-sdk fix adding a server-driven WS heartbeat to the
managed-process relay, which prevents most drops in the first place.

* style(mcp): ruff format

* chore(deps): pin langbot-plugin 0.4.8 for the managed-process WS heartbeat fix

---------

Co-authored-by: dadachann <185672915+dadachann@users.noreply.github.com>

pyproject.toml

@@ -70,7 +70,7 @@ dependencies = [
     "chromadb>=1.0.0,<2.0.0",
     "qdrant-client (>=1.15.1,<2.0.0)",
     "pyseekdb==1.1.0.post3",
-    "langbot-plugin==0.4.7",
+    "langbot-plugin==0.4.8",
     "asyncpg>=0.30.0",
     "line-bot-sdk>=3.19.0",
     "matrix-nio>=0.25.2",