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/heuristic_test.go

@@ -0,0 +1,142 @@
+package router
+
+import (
+	"testing"
+
+	"gophergate/internal/db"
+)
+
+func TestRouteHeuristic_ConditionRules(t *testing.T) {
+	targets := []string{
+		"deepseek-v4-flash", // index 0
+		"gemini-3-flash",    // index 1
+		"grok-build-0.1",    // index 2
+		"kimi-k2.6",         // index 3
+		"mimo-v2.5-pro",     // index 4
+		"grok-4.3",          // index 5
+		"deepseek-v4-pro",   // index 6
+	}
+
+	rulesJSON := `[
+		{
+			"rule_id": "fast_flow_extraction",
+			"conditions": {
+				"any_of_tags": ["fast-flow", "classification"],
+				"max_input_tokens_lt": 8000,
+				"requires_reasoning": false
+			},
+			"primary_model": "deepseek-v4-flash",
+			"fallback_model": "grok-build-0.1"
+		},
+		{
+			"rule_id": "multimodal_long_context",
+			"conditions": {
+				"any_of_tags": ["standard-pro", "long-doc"],
+				"has_multimodal_input": true
+			},
+			"primary_model": "gemini-3-flash",
+			"fallback_model": "mimo-v2.5-pro"
+		},
+		{
+			"rule_id": "regional_fallback_general",
+			"conditions": {
+				"is_default_fallback": true
+			},
+			"primary_model": "kimi-k2.6"
+		}
+	]`
+
+	group := db.ModelGroup{
+		ID:             "dustins_stack",
+		Strategy:       "heuristic",
+		HeuristicRules: &rulesJSON,
+	}
+
+	// 1. Test Match Fast Flow (condition success)
+	ctx1 := &RouteContext{
+		UserMessage:        "classify this JSON",
+		InputTokens:        500,
+		HasMultimodalInput: false,
+		RequiresReasoning:  false,
+		Tags:               []string{"fast-flow", "classification"},
+	}
+	dec1, err := routeHeuristic(group, targets, ctx1)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if dec1.SelectedModel != "deepseek-v4-flash" {
+		t.Fatalf("expected deepseek-v4-flash, got %s", dec1.SelectedModel)
+	}
+
+	// 2. Test Multimodal Long Context (condition success)
+	ctx2 := &RouteContext{
+		UserMessage:        "explain this video",
+		InputTokens:        15000,
+		HasMultimodalInput: true,
+		RequiresReasoning:  false,
+		Tags:               []string{"standard-pro", "video-analysis"},
+	}
+	dec2, err := routeHeuristic(group, targets, ctx2)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if dec2.SelectedModel != "gemini-3-flash" {
+		t.Fatalf("expected gemini-3-flash, got %s", dec2.SelectedModel)
+	}
+
+	// 3. Test Fallback general rule
+	ctx3 := &RouteContext{
+		UserMessage:        "hello there",
+		InputTokens:        100,
+		HasMultimodalInput: false,
+		RequiresReasoning:  false,
+		Tags:               []string{"general"},
+	}
+	dec3, err := routeHeuristic(group, targets, ctx3)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if dec3.SelectedModel != "kimi-k2.6" {
+		t.Fatalf("expected kimi-k2.6, got %s", dec3.SelectedModel)
+	}
+}
+
+func TestRouteHeuristic_LegacyRules(t *testing.T) {
+	targets := []string{"gpt-4o-mini", "deepseek-v4-pro", "kimi-k2.6"}
+
+	// Legacy pattern-based rule with regex
+	rulesJSON := `[
+		{"pattern": "\\b(agent|agents|tool use)\\b", "target": 1},
+		{"pattern": "summarize", "target": 2}
+	]`
+
+	group := db.ModelGroup{
+		ID:             "heavy-logic",
+		Strategy:       "heuristic",
+		HeuristicRules: &rulesJSON,
+	}
+
+	// 1. Test regex match
+	ctx1 := &RouteContext{
+		UserMessage: "We need an agent to do tool use",
+	}
+	dec1, err := routeHeuristic(group, targets, ctx1)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if dec1.SelectedModel != "deepseek-v4-pro" {
+		t.Fatalf("expected deepseek-v4-pro, got %s", dec1.SelectedModel)
+	}
+
+	// 2. Test literal match
+	ctx2 := &RouteContext{
+		UserMessage: "Please summarize this text",
+	}
+	dec2, err := routeHeuristic(group, targets, ctx2)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if dec2.SelectedModel != "kimi-k2.6" {
+		t.Fatalf("expected kimi-k2.6, got %s", dec2.SelectedModel)
+	}
+}