fix(host-agent): use stable ServerSession id for MCP lock identity

The previous _current_session_id() implementation tried to import a
non-existent get_context() helper, so the production code path always
fell through to the empty _TEST_SESSION_ID ContextVar — meaning every
MCP client shared the empty-string identity and there was no per-session
isolation in production.

Use Context.session (the long-lived ServerSession object) as the source
of identity. id(ctx.session) is stable across every tool call the same
client makes within a Streamable HTTP session, which is exactly what the
busy tracker needs to renew leases.

Wire FastMCP to inject the Context into the wrapper by setting
tool.context_kwarg = "ctx" after swapping tool.fn; wrap the swap in a
defensive try/except that surfaces a FastMcpSdkIncompatibilityError on
future SDK layout drift.

Add 4 tests covering the production path: stability across calls in the
same session, isolation between sessions, fallback to _TEST_SESSION_ID
when no Context is supplied, and verification that the registered tool
declares context_kwarg="ctx".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/device-host-agent/tests/test_web_mcp.py

@@ -13,8 +13,11 @@ from host_agent.status import AgentStatusTracker
 from host_agent.web.mcp import (
     McpDeviceBusyError,
     _call_tool_sync,
+    _current_session_id,
     build_mcp_server,
 )
+from mcp.server.fastmcp import Context
+from mcp.shared.context import RequestContext
 
 
 class _FakeDriver(Driver):
@@ -223,4 +226,85 @@ def test_manager_required_for_build_mcp_server() -> None:
     import inspect
 
     sig = inspect.signature(build_mcp_server)
-    assert sig.parameters["manager"].kind == inspect.Parameter.KEYWORD_ONLY
+    assert sig.parameters["manager"].kind == inspect.Parameter.KEYWORD_ONLY
+
+
+def _fake_ctx(session_obj: object) -> Context:
+    """Build a Context whose ``session`` attribute returns ``session_obj``.
+
+    Context's ``session`` is a property backed by ``request_context.session``;
+    we construct a minimal ``RequestContext`` and set it as the private
+    ``_request_context`` field. The pydantic public API doesn't expose a
+    setter for ``session``, so we use ``object.__setattr__`` on the private
+    backing field.
+    """
+    ctx = Context.model_construct()
+    request_ctx = RequestContext(
+        request_id="req-test",
+        meta=None,
+        session=session_obj,
+        lifespan_context=None,
+    )
+    object.__setattr__(ctx, "_request_context", request_ctx)
+    return ctx
+
+
+def test_current_session_id_is_stable_across_calls_same_session() -> None:
+    """Production-path identity: two tool calls from the same MCP session
+    must yield the same session_id so the busy tracker can renew the lease.
+
+    This exercises the ``Context.session`` code path (NOT the
+    ``_TEST_SESSION_ID`` fallback used by ``_call_tool_sync``)."""
+    sentinel_session = object()
+    ctx = _fake_ctx(sentinel_session)
+    first = _current_session_id(ctx)
+    second = _current_session_id(ctx)
+    assert first == second
+    assert first.startswith("mcp_session:")
+    # Object identity of the underlying ServerSession is the key — verifies
+    # we use id(ctx.session) rather than e.g. ctx.request_id.
+    assert first == f"mcp_session:{id(sentinel_session)}"
+
+
+def test_current_session_id_differs_across_sessions() -> None:
+    """Two different MCP sessions (distinct ServerSession objects) must
+    produce distinct session_ids so the busy tracker can isolate them."""
+    sess_a = object()
+    sess_b = object()
+    assert _current_session_id(_fake_ctx(sess_a)) != _current_session_id(
+        _fake_ctx(sess_b)
+    )
+
+
+def test_current_session_id_falls_back_when_no_context() -> None:
+    """When no Context is available (e.g. outside a FastMCP request lifecycle,
+    or via ``_call_tool_sync`` which omits the ctx kwarg), the test
+    contextvars override provides the session_id."""
+    token = None
+    try:
+        from host_agent.web import mcp as mcp_mod
+
+        token = mcp_mod._TEST_SESSION_ID.set("test-session-xyz")
+        assert _current_session_id(None) == "test-session-xyz"
+    finally:
+        if token is not None:
+            from host_agent.web import mcp as mcp_mod
+
+            mcp_mod._TEST_SESSION_ID.reset(token)
+
+
+def test_wrapped_tool_accepts_context_kwarg() -> None:
+    """The wrapper registered on FastMCP must declare a ``ctx`` parameter so
+    FastMCP injects the live Context (and ``tool.context_kwarg`` is set to
+    ``"ctx"``). Without this, FastMCP never injects context and we fall
+    back to the empty test default — the production bug this PR fixes."""
+    manager = _make_manager_with_device()
+    tracker = McpBusyTracker()
+    status = AgentStatusTracker()
+    server = build_mcp_server(
+        manager=manager, mcp_busy_tracker=tracker, status_tracker=status
+    )
+    tool_manager = server._tool_manager  # type: ignore[attr-defined]
+    tool = tool_manager.get_tool("take_screenshot")
+    assert tool is not None
+    assert tool.context_kwarg == "ctx"