fix(user): allow unblocking users promoted to admin

[loveulvu]

Fixes #39189.

IsUserBlockedBy intentionally treats admin users as not blocked, but CanUnblockUser was also using it to determine whether a blocking relationship exists. If a previously blocked user is later promoted to admin, the existing user_blocking record remains but can no longer be removed.

This change separates those two concerns by adding HasBlocking for checking the persisted blocking relationship. CanUnblockUser uses that relationship check while IsUserBlockedBy keeps its existing admin-user behavior.

A regression test verifies that an admin is still not considered blocked while an existing blocking relationship can still be unblocked.

Tests:

• go test ./models/user ./services/user -count=1

[Copilot]

🟢 Approval recommended

The change cleanly separates “effective blocked status” from “blocking relationship exists” and includes a targeted regression test for the reported failure mode.

Pull request overview

This PR fixes an edge case in the user blocking feature where a previously-blocked user promoted to admin could no longer be unblocked because admin users are intentionally treated as “not blocked” by IsUserBlockedBy.

Changes:

• Introduces user_model.HasBlocking to check for an existing persisted blocking relationship independent of admin status.
• Updates CanUnblockUser to use HasBlocking so unblock can proceed even if the blockee is now an admin.
• Adds a regression test ensuring admins are still treated as not blocked while existing block relationships remain removable.

File summaries

File	Description
services/user/block.go	Switches unblock eligibility to check the persisted relationship (HasBlocking) rather than effective blocked status (IsUserBlockedBy).
services/user/block_test.go	Adds regression coverage for unblock-after-promotion-to-admin behavior.
models/user/block.go	Refactors IsUserBlockedBy to delegate to new HasBlocking, preserving admin “not blocked” semantics.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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