vtgate: set ServerStatusAutocommit in handshake status flags

[lizztheblizz]

Description

Follow-up to #5825 / PR #6551, which fixed status flags on query response packets but not the initial handshake.

Sets SERVER_STATUS_AUTOCOMMIT in NewConnection() so the handshake packet reflects VTGate's default session state (Autocommit: true). Without this, clients that read the handshake flags (pymysql, Connector/J, etc.) misdetect the server as autocommit=OFF and may skip sending SET AUTOCOMMIT=0.

Backporting probably wouldn't hurt, but I'm not sure how to do that. 😅

Related Issue(s)

Fixes #19627

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

N/A

AI Disclosure

This PR was co-authored with Claude Code. The bug was identified through manual investigation of VTGate's MySQL protocol handshake behavior.

[vitess-bot]

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[mhamza15]

I've added the backport labels, so that'll handle backporting. Thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.48%. Comparing base (70c7a72) to head (7468bf4).
⚠️ Report is 73 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #19628       +/-   ##
===========================================
+ Coverage   69.67%   84.48%   +14.81%     
===========================================
  Files        1614      306     -1308     
  Lines      216793    54840   -161953     
===========================================
- Hits       151044    46332   -104712     
+ Misses      65749     8508    -57241     

Flag	Coverage Δ
partial	84.48% <100.00%> (?)

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[mhamza15]

@lizztheblizz You'll need to merge in main to pull in #19631 to fix the failing CI 🙇‍♂️