fix: honor auto fallback when model selection is unsupported

apps/backend/internal/agent/runtime/lifecycle/start_model.go

@@ -140,6 +140,13 @@ func applyStartModelPolicy(
 	decision.SetModelCalled = true
 	if err := applier.SetModel(ctx, policy.Model); err != nil {
 		if sessionmodel.IsMethodNotFound(err) {
+			if policy.AutoFallback {
+				log.Debug("agent does not support model selection, continuing on provider default",
+					zap.String("model", policy.Model), zap.Error(err))
+				decision = providerDefaultDecision(state, policy, ModelSelectionReasonSelectionUnsupported)
+				decision.SetModelCalled = true
+				return decision, nil
+			}
 			unsupported, unavailableErr := unavailableStartModel(state, policy, ModelSelectionReasonSelectionUnsupported)
 			unsupported.SetModelCalled = true
 			return unsupported, unavailableErr

apps/backend/internal/agent/runtime/lifecycle/start_model_executor_authority_test.go

@@ -79,6 +79,16 @@ func TestApplyStartModelPolicyExecutorAuthority(t *testing.T) {
 			wantCalls:     []string{"requested"},
 			wantErr:       `requested model "requested" is unavailable (reason: selection_unsupported)`,
 		},
+		{
+			name:          "auto fallback allows unsupported model selection",
+			state:         modelState("requested"),
+			policy:        StartModelPolicy{Model: "requested", AutoFallback: true},
+			applierErrors: []error{methodNotFoundErr()},
+			wantCalls:     []string{"requested"},
+			wantOutcome:   ModelSelectionOutcomeProviderDefault,
+			wantReason:    ModelSelectionReasonSelectionUnsupported,
+			wantWarning:   true,
+		},
 		{
 			name:        "auto fallback allows absent requested model",
 			state:       modelState("executor-default"),