Decouple external mount probing from guarded operations

[aby913]

Summary

• Add mounted-state change listeners so posix external mount guard can track GlobalMounted updates without package cycles.
• Replace request-scoped mount guard execution with background mount-root health probing and cached availability checks.
• Keep upload/get/list/create business operations outside the probe critical path so normal concurrency is not rejected by a mount-level probing flag.
• Map external mount unavailable errors in upload handlers to HTTP 503.

Testing

• go test ./pkg/global
• go test ./pkg/drivers/posix/posix (blocked: missing generated pkg/hertz/biz/model/api/{search,share,external} packages)
• go test ./pkg/hertz/biz/handler/upload (blocked: missing generated pkg/hertz/biz/model/... packages)