transaction_mode variable to return flag default if unset

[harshit-gangal]

Description

This PR fixes to return the flag default for transaction_mode instead of UNSPECIFIED value.

Related Issue(s)

• Fixes Default transaction_mode shows UNSPECIFIED but documentation says MULTI #8150

Checklist

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[vitess-bot]

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[hotjump]

@sougou hi, vitess have a plan to enhance distributed transaction in the roadmap. Vitess have supported 2pc in the past, and guarantee atomicity. Will vitess support Isolation and global transaction id in the future, and do you have more detailed plan to introduce? thanks.