prefetch: the prefetch buffer should be fully filled (#59496)

[ti-chi-bot]

This is an automated cherry-pick of #59496

What problem does this PR solve?

Issue Number: close #59495

Problem Summary:

What changed and how does it work?

use io.ReadFull to fully fill the buffer

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.

performance increased in a AWS manual test

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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-8.5@84f6a9a). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #59538   +/-   ##
================================================
  Coverage               ?   36.8692%           
================================================
  Files                  ?       1674           
  Lines                  ?     611735           
  Branches               ?          0           
================================================
  Hits                   ?     225542           
  Misses                 ?     364039           
  Partials               ?      22154           

Flag	Coverage Δ
integration	36.8692% <100.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	∅ <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	23.0647% <0.0000%> (?)

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[ti-chi-bot]

@ti-chi-bot: The following test 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/unit-test	74bc02e	link	true	/test unit-test

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[D3Hunter]

manually picked by #62061