GetIndexesQuery: Exclude expression indexes at the database level. Fix #1188.

[fdcastel]

FbDatabaseModelFactory.GetIndexes uses table.Columns.Single(...) to look up index columns by name. Expression-based indexes (e.g., COMPUTED BY (upper(col))) don't have real column entries in RDB$INDEX_SEGMENTS — their segment names don't match any table column — causing Single() to throw "Sequence contains no matching element".

Changes

FbDatabaseModelFactory.cs:

1. SQL query — Added AND i.RDB$EXPRESSION_SOURCE IS NULL to GetIndexesQuery to exclude expression indexes at the database level. Expression indexes can't be meaningfully represented in EF Core's scaffolded model anyway.
2. Code safety — Added a null/empty check for the COLUMNS value before iterating, using reader.IsDBNull(3) and string.IsNullOrEmpty() to skip indexes with no segments gracefully.

ScaffoldingTests.cs:

• ExpressionIndexDoesNotBreakScaffolding — Creates a table with an expression index and verifies scaffolding completes without throwing, and the expression index is excluded from the model.
• RegularIndexScaffoldedAlongsideExpressionIndex — Creates a table with both a regular and an expression index, verifying the regular index is scaffolded correctly while the expression index is excluded.