Fix many-to-many relationship queries with custom schema linking objects

[Copilot]

Why make this change?

Closes #3071

GraphQL queries with M:N relationships fail with SQL error 208 ("Invalid object name") when the linking object uses a custom schema (e.g., "linkingObject": "mySchema.books_authors"). The query works only if a table with the same name exists in dbo.

What is this change?

DatabaseObject.Equals() and GetHashCode() used case-sensitive string comparison. SQL Server schema/table names are case-insensitive, causing FK definition lookups to fail when database-returned names differ in casing from config-specified names.

• DatabaseObject.cs: Use StringComparison.OrdinalIgnoreCase for Equals() and ToUpperInvariant() for GetHashCode()

public bool Equals(DatabaseObject? other)
{
    return other is not null &&
           string.Equals(SchemaName, other.SchemaName, StringComparison.OrdinalIgnoreCase) &&
           string.Equals(Name, other.Name, StringComparison.OrdinalIgnoreCase);
}

How was this tested?

• Integration Tests
• Unit Tests

Added TestRelationshipWithLinkingObjectInCustomSchema with test cases for:

• Custom schema linking object (mySchema.TEST_SOURCE_LINK)
• Uppercase schema (MYSCHEMA.TEST_SOURCE_LINK)
• Lowercase schema and table (myschema.test_source_link)

Sample Request(s)

query {
  authors {
    items {
      id
      books {
        items {
          title
        }
      }
    }
  }
}

With config:

"authors": {
  "relationships": {
    "books": {
      "cardinality": "many",
      "target.entity": "books",
      "linking.object": "mySchema.books_authors",
      "linking.source.fields": ["author_id"],
      "linking.target.fields": ["book_id"]
    }
  }
}

Original prompt

work on the github issue #3071

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