feat: implement advanced condition-based heuristic model routing

Upgrades the routing engine to support tag, token limit, multimodal, reasoning, and tool calling conditions. Adds unit tests for the new routing features.

internal/router/router.go

@@ -16,6 +16,16 @@ type Decision struct {
 	Reason        string `json:"reason"`
 }
 
+// RouteContext holds metadata of the request to evaluate condition rules.
+type RouteContext struct {
+	UserMessage         string   `json:"user_message"`
+	InputTokens         int      `json:"input_tokens"`
+	HasMultimodalInput  bool     `json:"has_multimodal_input"`
+	RequiresToolCalling bool     `json:"requires_tool_calling"`
+	RequiresReasoning   bool     `json:"requires_reasoning"`
+	Tags                []string `json:"tags"`
+}
+
 // ClassifierFunc is the callback for classifier-based routing.
 type ClassifierFunc func(ctx context.Context, selectorModel, systemPrompt, userMessage string) (string, error)
 
@@ -53,7 +63,7 @@ func (r *Router) IsGroup(modelID string) bool {
 }
 
 // Route resolves a group to a concrete model.
-func (r *Router) Route(ctx context.Context, groupID string, userMessage string) (*Decision, error) {
+func (r *Router) Route(ctx context.Context, groupID string, routeCtx *RouteContext) (*Decision, error) {
 	group, ok := r.groups[groupID]
 	if !ok {
 		return nil, fmt.Errorf("unknown model group: %s", groupID)
@@ -66,12 +76,12 @@ func (r *Router) Route(ctx context.Context, groupID string, userMessage string)
 
 	switch group.Strategy {
 	case "heuristic":
-		return routeHeuristic(group, targets, userMessage)
+		return routeHeuristic(group, targets, routeCtx)
 	case "classifier":
 		if r.classify == nil {
-			return routeHeuristic(group, targets, userMessage)
+			return routeHeuristic(group, targets, routeCtx)
 		}
-		return routeClassifier(ctx, r.classify, group, targets, userMessage)
+		return routeClassifier(ctx, r.classify, group, targets, routeCtx)
 	default:
 		return nil, fmt.Errorf("unknown strategy: %s", group.Strategy)
 	}
@@ -80,7 +90,7 @@ func (r *Router) Route(ctx context.Context, groupID string, userMessage string)
 // RouteToConcrete resolves a model name to a concrete model, following group
 // chains recursively until a non-group target is reached. Returns the original
 // name unchanged if it is not a group.
-func (r *Router) RouteToConcrete(ctx context.Context, modelID string, userMessage string) (*Decision, error) {
+func (r *Router) RouteToConcrete(ctx context.Context, modelID string, routeCtx *RouteContext) (*Decision, error) {
 	const maxDepth = 10
 	visited := make(map[string]bool)
 	current := modelID
@@ -109,7 +119,7 @@ func (r *Router) RouteToConcrete(ctx context.Context, modelID string, userMessag
 		}
 		visited[current] = true
 
-		decision, err := r.Route(ctx, current, userMessage)
+		decision, err := r.Route(ctx, current, routeCtx)
 		if err != nil {
 			return nil, err
 		}