Make JsonApiFilterParser.s_separator private and immutable

[erlendellefsen]

JsonApiToolkit/Parsing/JsonApiFilterParser.cs:15 exposes public static readonly string[] s_separator. The array contents are mutable, so any caller can change them and break filter parsing for the whole process. The s_ prefix on a public field also breaks the editorconfig naming rule.

Acceptance criteria

• Field is private (or replaced with a private const string)
• No way to mutate it from outside the class
• All call sites updated, tests pass