fix: transaction_isolation to be applied at session level

[harshit-gangal]

Description

The PR #11987 that added support for transaction_isolation works as expected with the settings pool, but without it, the transaction_isolation level was applied on the next transaction by using set @@transaction_isolation instead it should be applied with set transaction_isolation so that it is applied at the session level. This only happens in the case when any shard session was already open.

go/vt/vtgate/engine/set.go

	// Update existing shard session with new system variable settings.
	rss := vcursor.Session().ShardSession()
        ...
	for i := 0; i < len(rss); i++ {
                ...
		fmt.Sprintf("set @@%s = %s", svs.Name, svs.Expr)
	}
        ...

This PR fixes it.

Related Issue(s)

• support transaction isolation modification through reserved connection system settings #11987

Checklist

• "Backport to:" labels have been added if this change should be back-ported 16.0
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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