scheduler(ticdc): fix impossible spans panic

[overvenus]

What problem does this PR solve?

Issue Number: close #7931
ref #7720

What is changed and how it works?

Fix panic cause by batch add table rate limit.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)
  See cdc continues panic with schedulerv3: impossible spans #7931

Questions

Will it cause performance regression or break compatibility?

No.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3AceShowHand
• sdojjy

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[sdojjy]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5742eb7

[ti-chi-bot]

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At the same time I will also trigger all tests for you:

/run-all-tests

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