sqlreplay: fix replay quits immediately after graceful pause

[djshow832]

What problem does this PR solve?

Issue Number: close #996

Problem Summary:
The graceful flag was not reset after start, leading to the replayer stops immediately at the second replay.

What is changed and how it works:

• Reset the graceful flag before start
• Add more logs to troubleshoot

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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

None

[ti-chi-bot]

Skipping CI for Draft Pull Request.
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@704568c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/sqlreplay/replay/replay.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #995   +/-   ##
=======================================
  Coverage        ?   67.55%           
=======================================
  Files           ?      140           
  Lines           ?    14179           
  Branches        ?        0           
=======================================
  Hits            ?     9579           
  Misses          ?     3917           
  Partials        ?      683           

Flag	Coverage Δ
unit	67.55% <88.88%> (?)

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[YangKeao]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YangKeao

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Needs approval from an approver in each of these files:

• OWNERS [YangKeao]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-11-28 03:03:41.681211057 +0000 UTC m=+844785.330405514: ☑️ agreed by YangKeao.