fix(migration): make SchemaMismatchException.TotalMissingCount lazy

[joshsmithxrm]

Summary

TotalMissingCount in SchemaMismatchException is calculated in the constructor and could become stale.

Problem

In SchemaMismatchException.cs:30:

public SchemaMismatchException(...)
{
    MissingColumns = missingColumns;
    TotalMissingCount = missingColumns.Values.Sum(list => list.Count);  // Calculated once
}

If the dictionary is mutated after construction, TotalMissingCount becomes stale.

Solution

Calculate in property getter instead of constructor:

public int TotalMissingCount => MissingColumns.Values.Sum(list => list.Count);

Files to Modify

• src/PPDS.Migration/Exceptions/SchemaMismatchException.cs

Work Stream

Stream B - Code Patterns

Acceptance Criteria

• Property calculates on access
• Behavior unchanged for normal usage
• All existing tests pass