…cts (#3106)

## Why make this change?

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 `StringComparer.OrdinalIgnoreCase.GetHashCode()` for
`GetHashCode()`

```csharp
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
- [x] 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`)

The test uses case-mismatched `DatabaseTable` instances (linking table
with lowercase, source tables with uppercase) to ensure the test fails
if `DatabaseTable.Equals` becomes case-sensitive again. Additionally,
`Verify(..., Times.Never)` assertions guard against the original
dbo-fallback regression by asserting `VerifyForeignKeyExistsInDB` is
never called with a `DatabaseTable` whose `SchemaName` is `dbo`.

## Sample Request(s)

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

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

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<summary>Original prompt</summary>

> work on the github issue #3071

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