Allow plugin subagent runs to pass provider/model overrides through the typed runtime, gateway validation, and ingress agent path. Apply explicit per-run overrides without persisting them to session state, and cover the gateway + agent behavior with regressions.
Regeneration-Prompt: |
PR #48277 already forwarded provider and model from plugin subagent runtime calls, but review found two contract gaps: the typed plugin runtime did not declare those fields, and the gateway agent RPC rejected them because the schema and handler path did not accept or propagate them. Make the override path end-to-end valid without broad refactors.
Keep the change additive. Extend the plugin runtime and gateway agent request types just enough to carry provider/model overrides. Ensure gateway ingress passes those fields into the existing agent command path, and let agent runs honor explicit per-call overrides without persisting them as session overrides. Add focused regression coverage for the gateway forwarding path and for non-persistent per-run overrides.
