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>

tests/unit_tests/provider/test_mcp_box_integration.py

@@ -639,10 +639,13 @@ class TestGetRuntimeInfoDict:
         assert info['box_session_id'] == 'mcp-shared'
         assert info['box_enabled'] is True
 
-    def test_transient_test_session_is_isolated_from_shared(self, mcp_module):
-        """A transient test session (config-page "test", no persisted UUID)
-        must NOT share the live "mcp-shared" Box session. Regression: a failing
-        test churned the shared session and tore down healthy live servers."""
+    def test_transient_test_shares_session_but_isolated_by_process(self, mcp_module):
+        """A transient config-page "test" now shares the same 'mcp-shared' Box
+        session as live servers (so a test reuses the running container / live
+        process instead of a cold per-test session bootstrap). Isolation is at
+        the PROCESS level: the test runs under its own process_id and only ever
+        stops that process_id, so it cannot disturb another server's live
+        process or the shared session itself."""
         ap = _make_ap()
         ap.box_service.available = True
         transient = _make_session(
@@ -670,10 +673,12 @@ class TestGetRuntimeInfoDict:
         )
         assert transient.is_transient is True
         assert live.is_transient is False
-        # Isolated session id for the test, shared for the live server.
-        assert transient._build_box_session_id() == 'mcp-test-gen-uuid-123'
+        # Both share ONE Box session ...
+        assert transient._build_box_session_id() == 'mcp-shared'
         assert live._build_box_session_id() == 'mcp-shared'
-        assert transient._build_box_session_id() != live._build_box_session_id()
+        assert transient._build_box_session_id() == live._build_box_session_id()
+        # ... but are isolated by distinct process_ids within that session.
+        assert transient._box_stdio_runtime.process_id != live._box_stdio_runtime.process_id
 
     def test_stdio_session_refuses_when_box_unavailable(self, mcp_module):
         """Policy: when Box is configured but unavailable (disabled in config