Pinned Cursor Claude/GPT thinking variants returned an empty turn (502
'Provider returned empty content') because OmniRoute encoded the model
only as RequestedModel (AgentRunRequest field 9) with a bare model_id.
cursor-agent's working wire format also sends a ModelDetails envelope
(field 3) with model_id + display_model_id + display_name; the pinned
thinking variants need it to resolve. Add ModelDetails additively,
keeping RequestedModel (and its -fast parameters) so server-routed ids
(auto/composer) are unaffected.
Closes#3714
969addef6b