Exported for custom resolveFrodoForRequest implementations (see
McpToolRuntimeOptions.resolveFrodoForRequest) that reuse a
pre-authenticated singleton Frodo instance for performance instead of
calling resolveRequestScopedFrodo on every request: such a
resolver still needs to know when context carries a realm that differs
from the singleton's own, so it can fall back to a genuinely scoped
instance for that one call rather than silently ignoring the override.
Reads the effective realm from a runtime request context.