proxy: check the packet length of handshake response is greater or equal than 32

[YangKeao]

What problem does this PR solve?

Issue Number: close #952

Problem Summary:

Previously, a malformed packet will cause panic (though in recoverable goroutine). It'll increase metrics.ServerErrCounter.WithLabelValues("panic") and cause some alerts.

What is changed and how it works:

I checked that the length of the handshake response packet (or ssl request packet) should be greater or equal than 32, so some of the malformed packet will be refused.

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@5a50d72). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #956   +/-   ##
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  Coverage        ?   67.60%           
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  Files           ?      136           
  Lines           ?    13479           
  Branches        ?        0           
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  Hits            ?     9112           
  Misses          ?     3739           
  Partials        ?      628           

Flag	Coverage Δ
unit	67.60% <100.00%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: djshow832

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• OWNERS [djshow832]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-23 09:58:44.63502572 +0000 UTC m=+952230.712278270: ☑️ agreed by djshow832.

[YangKeao]

/cherry-pick release-nextgen-20251023

[ti-chi-bot]

@YangKeao: new pull request created to branch release-nextgen-20251023: #958.

In response to this:

/cherry-pick release-nextgen-20251023

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