Naming cleanup: constants, namespace style, const prefix

[erlendellefsen]

A few naming rule violations have crept in.

Acceptance criteria

• MIN_PAGE_SIZE in JsonApiQueryParser.cs:16 renamed to MinPageSize
• s_jsonApiMediaType const in JsonApiContentTypeFilter.cs:11 renamed to JsonApiMediaType
• ServiceCollectionExtensions.cs:14 converted from block-scoped to file-scoped namespace
• .editorconfig adds csharp_style_namespace_declarations = file_scoped:warning
• dotnet_naming_rule.constant_fields_should_be_pascal_case.severity bumped from suggestion to warning