Fix impossible type assertion in Remove method

[Copilot]

The Remove method attempted to type assert Instance interface to *gracefulServer, but gracefulServer doesn't implement Instance (missing Addr() method). This caused go vet and linter failures.

Changes:

• Changed type assertion from instance.(*gracefulServer) to instance.(*serverInstance)
• Updated field access to si.gracefulServer.shutdownTimeout to traverse the wrapper struct
• Added nil check for si.gracefulServer before accessing shutdownTimeout

// Before (broken)
if gs, ok := instance.(*gracefulServer); ok && gs.shutdownTimeout > 0 {
    timeout = gs.shutdownTimeout
}

// After (fixed)
if si, ok := instance.(*serverInstance); ok && si.gracefulServer != nil && si.gracefulServer.shutdownTimeout > 0 {
    timeout = si.gracefulServer.shutdownTimeout
}

The serverInstance struct properly implements the Instance interface and wraps gracefulServer internally.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.