feat(config): add support for Fork in OAuth model mappings with alias handling

Implemented `Fork` flag in `ModelNameMapping` to allow aliases as additional models while preserving the original model ID. Updated the `applyOAuthModelMappings` logic, added tests for `Fork` behavior, and updated documentation and examples accordingly.

sdk/cliproxy/service.go

@@ -1231,7 +1231,13 @@ func applyOAuthModelMappings(cfg *config.Config, provider, authKind string, mode
 	if len(mappings) == 0 {
 		return models
 	}
-	forward := make(map[string]string, len(mappings))
+
+	type mappingEntry struct {
+		alias string
+		fork  bool
+	}
+
+	forward := make(map[string]mappingEntry, len(mappings))
 	for i := range mappings {
 		name := strings.TrimSpace(mappings[i].Name)
 		alias := strings.TrimSpace(mappings[i].Alias)
@@ -1245,7 +1251,7 @@ func applyOAuthModelMappings(cfg *config.Config, provider, authKind string, mode
 		if _, exists := forward[key]; exists {
 			continue
 		}
-		forward[key] = alias
+		forward[key] = mappingEntry{alias: alias, fork: mappings[i].Fork}
 	}
 	if len(forward) == 0 {
 		return models
@@ -1260,10 +1266,45 @@ func applyOAuthModelMappings(cfg *config.Config, provider, authKind string, mode
 		if id == "" {
 			continue
 		}
-		mappedID := id
-		if to, ok := forward[strings.ToLower(id)]; ok && strings.TrimSpace(to) != "" {
-			mappedID = strings.TrimSpace(to)
+		key := strings.ToLower(id)
+		entry, ok := forward[key]
+		if !ok {
+			if _, exists := seen[key]; exists {
+				continue
+			}
+			seen[key] = struct{}{}
+			out = append(out, model)
+			continue
 		}
+		mappedID := strings.TrimSpace(entry.alias)
+		if mappedID == "" {
+			if _, exists := seen[key]; exists {
+				continue
+			}
+			seen[key] = struct{}{}
+			out = append(out, model)
+			continue
+		}
+
+		if entry.fork {
+			if _, exists := seen[key]; !exists {
+				seen[key] = struct{}{}
+				out = append(out, model)
+			}
+			aliasKey := strings.ToLower(mappedID)
+			if _, exists := seen[aliasKey]; exists {
+				continue
+			}
+			seen[aliasKey] = struct{}{}
+			clone := *model
+			clone.ID = mappedID
+			if clone.Name != "" {
+				clone.Name = rewriteModelInfoName(clone.Name, id, mappedID)
+			}
+			out = append(out, &clone)
+			continue
+		}
+
 		uniqueKey := strings.ToLower(mappedID)
 		if _, exists := seen[uniqueKey]; exists {
 			continue