Fix EF double-write in InterviewSessionRepository

[KhawarHabibKhan]

Fixes the double-write anti-pattern in InterviewSessionRepository where both ExecuteUpdateAsync and SaveChangesAsync are called on the same entity, causing two database round-trips and a race condition window for concurrent modifications.

Also fixes a related issue where the transcript StringBuilder logic produced leading whitespace/blank lines on the first update.

What Changed

• Removed SaveChangesAsync() from UpdateInterviewSessionAsync and CompleteInterviewSessionAsync
• Added EntityState.Detached after ExecuteUpdateAsync to prevent stale tracked state
• Replaced StringBuilder transcript concatenation with clean string interpolation
• Removed unused System.Text import

[KhawarHabibKhan]

@microsoft-github-policy-service agree

[KhawarHabibKhan]

please review the changes.