fix: keep health check when proxy group is named default

[soffchen]

Description

A fallback / url-test / load-balance proxy group named default never runs its periodic health check. Traffic stays on the first node until someone triggers a manual delay test.

This was found on a gateway running Mihomo Meta v1.19.30: a fallback group named default with interval: 30 did not URL-test for hours. After a restart it still selected the first node with empty delay history. GET /group/default/delay immediately marked dead nodes and switched. Renaming the group to proxy made the 30s ticker start on boot.

proxy-providers cannot be named default (ReservedName), but a proxy-group of the same name is allowed. ParseProxyGroup registers that group's compatible provider (with url / interval) as providersMap["default"]. parseProxies then overwrites the map entry with the reserved compatible provider used by GLOBAL (url == "", interval == 0). loadProvider only Initial()s map entries, so the group's health-check goroutine never starts. Untested nodes are treated as alive, so fallback sticks to the first member.

Change

Create the reserved provider for GLOBAL as before. If a user group already claimed providersMap["default"], keep that entry and only attach the reserved provider to GLOBAL. Configs without a group named default are unchanged.

Reproduction

proxies:
  - { name: node-a, type: socks5, server: 127.0.0.1, port: 1080 }
  - { name: node-b, type: socks5, server: 127.0.0.1, port: 1081 }
proxy-groups:
  - name: default
    type: fallback
    url: http://www.gstatic.com/generate_204
    interval: 30
    proxies: [node-a, node-b]

After startup, GET /providers/proxies/default shows an empty testUrl and the reserved all-outbound list, not the group's members. Delay history stays empty until a manual group delay test.

Validation

gofmt -l config/config.go config/utils_test.go
git diff --check
go test ./config/

[wwqgtxx]

This is essentially an implementation oversight; users shouldn't be allowed to define a group named "default".