snap_backup: make snapshot backup exit once importing task detected

[YuJuncen]

What problem does this PR solve?

Issue Number: ref #46850

Problem Summary:
See the issue.

What is changed and how it works?

Snapshot backup cannot be compatible with the importing for now. We will reject to start ebs backup once there are any importing tasks. (ADD INDEX by importing isn't covered yet!)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  I think we will test this via some automation then.
  When backup full --type aws-ebs is running, try to start tide-lightning:

When tidb-lightning is running, try to start backup via snapshot:

• No code

Release note

Added a check to reject some scenarios possible to creating broken backup for snapshot backup.

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[BornChanger]

Blocking here is improper. We'd better to keep move it to the task of suspend gc and schedule phase. Then we can release as soon as possible.

[YuJuncen]

Close in favor of #47001