feat(mcp): add box.default_memory_mb config for nsjail memory limit (#2318)

Operators can now set a global default memory limit for all stdio MCP
servers in config.yaml or via environment variable:

  config.yaml:
    box:
      default_memory_mb: 2048  # default: 1536

  env:
    BOX__DEFAULT_MEMORY_MB=2048

The default is raised from 1024 to 1536 MB — a safer floor for
Node.js V8 + WASM (undici llhttp) under nsjail cgroup limits.
Individual MCP servers can still override via their own box.memory_mb.

Previously the fallback was hardcoded to 1024 MB, causing OOM kills
(return_code=137) on node/npx MCP servers that need more RAM.

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

tests/unit_tests/provider/test_mcp_box_integration.py

@@ -417,7 +417,7 @@ class TestBuildBoxSessionPayload:
         payload = s._build_box_session_payload('session-123')
         assert payload['image'] == 'node:20'
         assert payload['cpus'] == 2.0
-        assert payload['memory_mb'] == 1024
+        assert payload["memory_mb"] == 1024
         assert payload['pids_limit'] == 256
 
     def test_none_fields_excluded(self, mcp_module):