feat(mssql): expose completion context scope

[rebelice]

Summary

• Add MSSQL parser-native CollectCompletion context with prefix, intent, CTEs, and visible scope snapshots.
• Track DML target, MERGE target/source, CTE refs, outer refs, set-op current arm scope, and VALUES derived table alias columns.
• Route MSSQL completion table-ref extraction through the parser completion scope to avoid set-op reference leakage.

Test Plan

• go test ./mssql/... -count=1