Built-in actors generated from the provider registry have a config_blob
with provider and model fields but no type field. When this raw blob is
serialised to YAML and fed to ReactiveConfigParser, the parser produces
an empty ReactiveConfig with no agents and no routes, causing
run_single_shot() to return empty string (bug #10861).
Fix: resolve_config_files now synthesises a minimal v3 type: llm YAML
when the actor has no yaml_text and the config_blob has provider and
model but no type field. This allows the reactive config parser to
create a working agent and graph route, enabling the LLM to be invoked
and its response returned to the caller.
ISSUES CLOSED: #10861
