refactor(mcp): make MCP test reuse the shared Box session (near-instant tests) (#2304)

* refactor(mcp): make MCP test reuse the shared Box session instead of a per-test session

Testing an MCP server (config-page "test" button) previously spun up a fresh
isolated mcp-test-<uuid> Box session every time: cold-start the container, run
the dependency bootstrap, probe, then tear the whole session down. That is slow
(tens of seconds) and, on an already-hosted server, wasteful — the server is
already running in the shared session.

Change the test to reuse the shared session / live process:

- _build_box_session_id: transient tests now use mcp-shared, the same Box
  session as live servers, so a test reuses the running container (and, for an
  existing server, its live managed process) instead of a cold per-test session.
- cleanup_session: a transient test no longer deletes the whole session (which
  under the shared model would kill every other MCP server in the container). It
  stops only its own process_id, exactly like a live server. Isolation is now at
  the process level (distinct process_id per server/test), not the session level.
- test_mcp_server (persisted server): reuse the live connection with a real
  list_tools refresh/probe; only fall back to a full start() when there is no
  live connection to probe or the refresh fails, instead of an ERROR->start()
  rebuild.

Trade-off: a failing test now shares the container with live servers rather than
a throwaway session. Accepted deliberately in favour of near-instant tests;
process-level isolation keeps a test from stopping another server's process.

* chore(deps): pin langbot-plugin 0.4.9 for the nsjail RLIMIT_AS node/npx MCP fix

---------

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

src/langbot/pkg/provider/tools/loaders/mcp_stdio.py

@@ -370,16 +370,20 @@ class BoxStdioSessionRuntime:
 
         workspace = self._build_workspace(host_path=None)
 
-        # Transient test sessions own their isolated Box session, so tear the
-        # whole session down rather than leaking it. This cannot affect live
-        # servers because they live in the separate shared session.
+        # Transient config-page tests now share the same 'mcp-shared' Box
+        # session as live servers, so we must NOT tear the session down here —
+        # that would kill every other MCP server in the container. A test is
+        # isolated at the process level: it ran under its own process_id, so we
+        # stop only that process, exactly like a live server does below. The
+        # shared session and all other servers' live processes are untouched.
+        # (Staged per-test workspace files are still cleaned up.)
         if getattr(self.owner, 'is_transient', False):
             try:
-                await workspace.cleanup()
+                await workspace.stop_managed_process(self.process_id)
             except Exception as exc:
                 self.ap.logger.warning(
-                    f'MCP server {self.server_name}: failed to delete transient test session '
-                    f'{self.owner._build_box_session_id()}: {type(exc).__name__}: {exc}'
+                    f'MCP server {self.server_name}: failed to stop transient test process '
+                    f'process_id={self.process_id}: {type(exc).__name__}: {exc}'
                 )
             await self._cleanup_staged_workspace()
             return