br: handle the merged SST response

[Leavrth]

What problem does this PR solve?

Issue Number: close #57389

Problem Summary:
the backup merge data from different key ranges into one SST file, so BR needs to be able to handle its response.

What changed and how does it work?

handle the merged SST response.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[tiprow]

Hi @Leavrth. Thanks for your PR.

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[ti-chi-bot]

@Leavrth: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev_2	a5b92cf	link	true	/test check-dev2
idc-jenkins-ci-tidb/mysql-test	a5b92cf	link	true	/test mysql-test
idc-jenkins-ci-tidb/unit-test	a5b92cf	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	a5b92cf	link	true	/test check-dev
pull-integration-ddl-test	a5b92cf	link	true	/test pull-integration-ddl-test
pull-br-integration-test	a5b92cf	link	true	/test pull-br-integration-test
pull-integration-e2e-test	a5b92cf	link	true	/test pull-integration-e2e-test
pull-mysql-client-test	a5b92cf	link	true	/test pull-mysql-client-test
idc-jenkins-ci-tidb/build	a5b92cf	link	true	/test build

Full PR test history. Your PR dashboard.

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