sqlreplay, backend: reconnect after a replaying connection disconnects

[djshow832]

What problem does this PR solve?

Issue Number: close #964

Problem Summary:
There are 2 problems when generating InitDB when parsing:

• The CPS may differ from the production cluster because the production cluster generally inits DB in handshake
• When a connection disconnects, the subsequent commands in the connection are discarded, resulting in QPS drop during TiDB restarting.

So the better way is to initializing DB in the handshake when the backend connection creates or reconnects.

What is changed and how it works:

• Do not generate InitDB command in the decode phase, set the CurDB in the command instead
• In the replay phase, handshake with the initial DB or generate InitDB command if necessary
• Remove the waitForQuitMode, reconnect before the next command if the connection disconnects
• If a prepared statement is not found, do not send it to the server, just generate an exception

Check List

Tests

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

1. replay
2. kill tidb and start another tidb
3. check the failure count

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.96610% with 13 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@358165b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/sqlreplay/conn/conn.go	81.08%	6 Missing and 1 partial ⚠️
pkg/sqlreplay/conn/backend_conn.go	45.45%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage        ?   67.46%           
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  Files           ?      136           
  Lines           ?    13633           
  Branches        ?        0           
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  Hits            ?     9197           
  Misses          ?     3795           
  Partials        ?      641           

Flag	Coverage Δ
unit	67.46% <77.96%> (?)

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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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• OWNERS [YangKeao]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-11-06 15:01:21.586882813 +0000 UTC m=+368731.029912702: ☑️ agreed by YangKeao.