feat(gemini-web): Add conversation affinity selector

internal/provider/gemini-web/conversation/lookup.go

@@ -0,0 +1,40 @@
+package conversation
+
+import "strings"
+
+// PrefixHash represents a hash candidate for a specific prefix length.
+type PrefixHash struct {
+	Hash      string
+	PrefixLen int
+}
+
+// BuildLookupHashes generates hash candidates ordered from longest to shortest prefix.
+func BuildLookupHashes(model string, msgs []Message) []PrefixHash {
+	if len(msgs) < 2 {
+		return nil
+	}
+	model = NormalizeModel(model)
+	sanitized := SanitizeAssistantMessages(msgs)
+	result := make([]PrefixHash, 0, len(sanitized))
+	for end := len(sanitized); end >= 2; end-- {
+		tailRole := strings.ToLower(strings.TrimSpace(sanitized[end-1].Role))
+		if tailRole != "assistant" && tailRole != "system" {
+			continue
+		}
+		prefix := sanitized[:end]
+		hash := HashConversationGlobal(model, ToStoredMessages(prefix))
+		result = append(result, PrefixHash{Hash: hash, PrefixLen: end})
+	}
+	return result
+}
+
+// BuildStorageHashes returns hashes representing the full conversation snapshot.
+func BuildStorageHashes(model string, msgs []Message) []PrefixHash {
+	if len(msgs) == 0 {
+		return nil
+	}
+	model = NormalizeModel(model)
+	sanitized := SanitizeAssistantMessages(msgs)
+	hash := HashConversationGlobal(model, ToStoredMessages(sanitized))
+	return []PrefixHash{{Hash: hash, PrefixLen: len(sanitized)}}
+}