[thanos] 5407: fix nested errors

[verejoel]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Verification

[Copilot]

Pull Request Overview

This PR enhances error classification in the receive handler by properly detecting nested and wrapped conflict errors, and adds corresponding unit tests.

• Expanded isConflict to unwrap errors, check root causes, and handle MultiError chains.
• Updated isNotReady and isUnavailable to use errors.Is and inspect wrapped causes.
• Added TestIsConflictWrappedErrors and TestReplicationErrorsWithWrappedConflicts to verify nested conflict detection.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/receive/handler.go	Expanded error-checking functions to handle wrapped and multi-errors for conflict, not-ready, and unavailable cases.
pkg/receive/handler_test.go	Added tests covering direct, wrapped, and nested conflict errors, plus replication error behavior.

Comments suppressed due to low confidence (1)

pkg/receive/handler.go:1161

• [nitpick] Consider adding unit tests for isNotReady (and similarly for isUnavailable) to verify wrapped errors and multi-error chains are correctly recognized by the new unwrapping logic.

func isNotReady(err error) bool {

[Copilot]

Consider using errors.Is(err, errConflict) instead of direct equality to correctly handle wrapped instances of the sentinel errConflict.

Suggested change

	if err == errConflict ||
	if errors.Is(err, errConflict) ||