feat: add runner failover rules and cache affinity

apps/api/internal/runner/cache_affinity.go

@@ -0,0 +1,69 @@
+package runner
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"strings"
+)
+
+func buildCacheAffinityKey(kind string, modelType string, body map[string]any) string {
+	if len(body) == 0 {
+		return ""
+	}
+	for _, key := range []string{"cacheAffinityKey", "cache_affinity_key"} {
+		if value := strings.TrimSpace(stringFromAny(body[key])); value != "" {
+			return "explicit:" + sha256Text(value)
+		}
+	}
+	for _, key := range []string{"sessionId", "session_id", "conversationId", "conversation_id"} {
+		if value := strings.TrimSpace(stringFromAny(body[key])); value != "" {
+			return key + ":" + sha256Text(value)
+		}
+	}
+	if !cacheAffinityPromptHashSupported(kind, modelType) {
+		return ""
+	}
+	payload := map[string]any{
+		"kind":      kind,
+		"modelType": modelType,
+	}
+	if messages, ok := body["messages"]; ok {
+		payload["messages"] = messages
+	}
+	if tools, ok := body["tools"]; ok {
+		payload["tools"] = tools
+	}
+	if instructions, ok := body["instructions"]; ok {
+		payload["instructions"] = instructions
+	}
+	if input, ok := body["input"]; ok {
+		payload["input"] = input
+	}
+	if len(payload) <= 2 {
+		return ""
+	}
+	raw, err := json.Marshal(payload)
+	if err != nil || len(raw) == 0 {
+		return ""
+	}
+	return "prompt:" + sha256Text(string(raw))
+}
+
+func cacheAffinityPromptHashSupported(kind string, modelType string) bool {
+	switch strings.TrimSpace(kind) {
+	case "chat.completions", "responses":
+		return true
+	}
+	switch strings.TrimSpace(modelType) {
+	case "chat", "text_generate", "responses":
+		return true
+	default:
+		return false
+	}
+}
+
+func sha256Text(value string) string {
+	sum := sha256.Sum256([]byte(value))
+	return hex.EncodeToString(sum[:])
+}