Handle dollar-prefixed columns in SourceEntityRelationshipMap during serialization

[Copilot]

Why make this change?

Addresses review feedback on #3051 where dollar-prefixed column names in ForeignKeyDefinition objects within SourceEntityRelationshipMap were not being escaped/unescaped during serialization/deserialization, causing inconsistencies in relationship metadata.

What is this change?

Extended the existing dollar-prefix escaping logic to handle relationship metadata:

• EscapeDollaredColumns/UnescapeDollaredColumns: Now traverse SourceEntityRelationshipMap → RelationshipMetadata → ForeignKeyDefinition and process ReferencedColumns and ReferencingColumns lists
• EscapeColumnList/UnescapeColumnList: New helper methods to handle List<string> collections with null safety
• Write method: Immediately restores original state after serialization (escape → serialize → unescape) to prevent mutation issues with circular references when using ReferenceHandler.Preserve

The escaping pattern remains unchanged: $ColumnName → DAB_ESCAPE$ColumnName during serialization, reversed during deserialization.

How was this tested?

• Unit Tests

Added TestSourceDefinitionRelationshipMapSerializationDeserialization_WithDollarColumn to verify dollar-prefixed columns in relationship metadata are properly escaped/unescaped. All 12 existing serialization/deserialization tests continue to pass.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.