Query Frontend: Handle context error before decoding and merging responses

[justinjung04]

What this PR does:

Decoding and merging response in query frontend can take a long time, and if the context has been cancelled in the process it should stop as well.

This PR adds context error check before heavy operations within decode and merge responses.

Which issue(s) this PR fixes:
n/a

Checklist

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

[yeya24]

Thanks!

[yeya24]

golangci-lint run
pkg/querier/tripperware/instantquery/instant_query_test.go:385:4: lostcancel: the cancelCtx function is not used on all paths (possible context leak) (govet)
			ctx, cancelCtx := context.WithCancel(context.Background())
			^
pkg/querier/tripperware/instantquery/instant_query_test.go:401:6: lostcancel: this return statement may be reached without using the cancelCtx var defined on line 385 (govet)
					return
					^
pkg/querier/tripperware/queryrange/query_range_test.go:114:4: lostcancel: the cancelCtx function is not used on all paths (possible context leak) (govet)
			ctx, cancelCtx := context.WithCancel(context.Background())
			^
pkg/querier/tripperware/queryrange/query_range_test.go:126:5: lostcancel: this return statement may be reached without using the cancelCtx var defined on line 114 (govet)
				return
				^
pkg/querier/tripperware/queryrange/query_range_test.go:[7](https://github.com/cortexproject/cortex/actions/runs/5765792485/job/15632449186?pr=5499#step:6:8)12:4: lostcancel: the cancelCtx function is not used on all paths (possible context leak) (govet)
			ctx, cancelCtx := context.WithCancel(context.Background())
			^
pkg/querier/tripperware/queryrange/query_range_test.go:71[9](https://github.com/cortexproject/cortex/actions/runs/5765792485/job/15632449186?pr=5499#step:6:10):5: lostcancel: this return statement may be reached without using the cancelCtx var defined on line 7[12](https://github.com/cortexproject/cortex/actions/runs/5765792485/job/15632449186?pr=5499#step:6:13) (govet)
				return
				^

Need to fix this by calling cancelCtx in all test cases