feat(mcp): per-request user_id contextvar for tool handlers

Adds mcp._context.current_user_id() backed by a ContextVar. The ASGI
auth middleware sets it before dispatching to FastMCP and resets it
on the way out, so tool handlers can read the acting user without
re-parsing the request scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_mcp_context.py

@@ -0,0 +1,25 @@
+"""Tests for MCP per-request user_id context."""
+import pytest
+
+from fabledassistant.mcp._context import current_user_id, _user_id_ctx
+
+
+def test_current_user_id_raises_when_unset():
+    """No active context — must raise so tools don't silently fall back to a default."""
+    _user_id_ctx.set(None)
+    with pytest.raises(RuntimeError, match="no MCP user context"):
+        current_user_id()
+
+
+def test_current_user_id_returns_set_value():
+    _user_id_ctx.set(42)
+    assert current_user_id() == 42
+
+
+def test_current_user_id_isolation_via_contextvar_token():
+    """Reset via the token returned by set() restores prior state."""
+    _user_id_ctx.set(1)
+    token = _user_id_ctx.set(2)
+    assert current_user_id() == 2
+    _user_id_ctx.reset(token)
+    assert current_user_id() == 1