restore: set config value to default when failed to get config from tikv (#36051)

[ti-chi-bot]

This is an automated cherry-pick of #36051

Signed-off-by: Yu Juncen yujuncen@pingcap.com

What problem does this PR solve?

Issue Number: close #36053

Problem Summary:
If we failed to get region size config from TiKV, we abort the restore procedure, which may not be robust.

What is changed and how it works?

Use the default config if the region size cannot be fetched from TiKV.

Check List

Tests

(TBD)

• Unit test
• Manual test (add detailed scripts or steps below)

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fixed a bug that may cause BR reject to restore unexpectedly.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• Leavrth

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[YuJuncen]

/merge

[ti-chi-bot]

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

Commit hash: 67313b4

[wuhuizuo]

/hold high priority for trunk branch

[ti-chi-bot]

/unhold

[wuhuizuo]

/retest

[Ivy-YinSu]

/test check-dev2
/test mysql-test

[YuJuncen]

/retest-required

[3pointer]

/test check-dev2

[ti-chi-bot]

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