Fix: reset transaction session when no reserved connection

[harshit-gangal]

Description

This PR fixes a bug exposed on PRS when the settings pool is enabled on vttablet.
A transaction leaves the tablet tagged to the shard session even after a commit or rollback.

On PRS, that session will keep sending the query to the same tablet and would receive the error until that session is closed and new session is created.
vttablet: rpc error: code = FailedPrecondition desc = wrong tablet type: PRIMARY, want: REPLICA or []", state: "HY000"

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported 16.0 and main
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[vitess-bot]

I was unable to backport this Pull Request to the following branches: release-16.0.

[vitess-bot]

I was unable to forwardport this Pull Request to the following branches: main.