Close per-query querier in the remote read handler

[anxkhn]

What this PR does:

The remote read handler creates a storage.Querier for each sub-query but never calls Close(). It is the only storage.Querier call site in Cortex that does not close the querier, so it does not honor the interface contract.

The concrete queriers in the current read path have Close() methods that return nil, so this is not an observed resource leak today. It is a correctness and consistency fix that ensures future or wrapped queriers which release resources are closed deterministically.

This adds defer querier.Close() immediately after successful creation, covering both success and error paths. A regression test verifies every created querier is closed.

Which issue(s) this PR fixes:

No existing issue; found by code inspection.

Checklist

• Tests updated
• Documentation added (n/a, no user-facing docs or flags changed)
• CHANGELOG.md updated
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags (n/a, no flags)

This change was prepared with AI assistance per the project policy. I reviewed every line, verified the interface and concrete implementations, and validated the tests and linters.

[siddarth2810]

Good catch, seems good to me.

One minor thing in the description, in tenant-federation merge queryable Close() returns just nil, so I'm thinking if its a weak example for "releases the resources of the Querier" ?

[anxkhn]

Good point. You are right that mergeQuerier.Close() is a weak example. I checked the concrete queriers in the current read path, and their Close() methods are effectively no-ops today, so the original description overstated this as an existing resource leak. I corrected the title, description, CHANGELOG, and commit message to frame this as honoring the storage.Querier contract and keeping the handler consistent with other call sites.

I also fixed both CI failures: the test now uses go.uber.org/atomic to satisfy faillint, and polls for deferred closes because they can finish just after ServeHTTP returns.