fix panic on health check failure when using stream push

[SungJin1212]

I've found a panic would be occur in below scenario, and added test case to show it.
This PR adds channel close check logic to prevent the panic.

Scenario

1. Ingester health check fail in pool
2. closableHealthAndIngesterClient.Close() called
3. streamPushChan closed
4. job is nil in case job := <-c.streamPushChan
5. get panic when err = stream.Send(job.req)

Related logs:

ts=2025-11-14T17:01:37.161148853Z caller=pool.go:184 level=warn msg="removing ingester failing healthcheck" addr=10.240.7.140:9095 reason="rpc error: code = DeadlineExceeded desc = context deadline exceeded"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x25f81cc]

goroutine 292091 [running]:
github.com/cortexproject/cortex/pkg/ingester/client.(*closableHealthAndIngesterClient).worker.func1()
	/__w/cortex/cortex/pkg/ingester/client/client.go:235 +0xec
created by github.com/cortexproject/cortex/pkg/ingester/client.(*closableHealthAndIngesterClient).worker in goroutine 291921
	/__w/cortex/cortex/pkg/ingester/client/client.go:229 +0x117

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

@alexqyle @danielblando Can you help take a look?