raft: scope join retry context to core shutdown lifecycle

[1seal]

summary

• scope raft join retry goroutines to the core shutdown lifecycle by passing a shutdown-scoped context into JoinRaftCluster.

background

/v1/sys/storage/raft/join currently passes context.Background() into JoinRaftCluster. when retry=true, JoinRaftCluster may spawn a retry loop that checks ctx.Done() but will never observe cancellation.

this change adds Core.ShutdownContext() (canceled in Core.Shutdown()) and uses it in the raft join http handler, so retries are stopped automatically when the core is shutting down.

testing

• go test ./http/... -run TestNonExistent
• go test ./vault/... -run TestNonExistent

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[1seal]

thanks for the review — good call to check.

ShutdownCoreError already routes through Shutdown via ShutdownWait:

func (c *Core) ShutdownCoreError(err error) {
    c.Logger().Error("shutting down core", "error", err)
    if shutdownErr := c.ShutdownWait(); shutdownErr != nil { // ← calls Shutdown()
        ...
    }
}

so shutdownCtxCancel() fires on that path too — no additional wiring needed.