Use raw string literals for multi-line strings

[craigsmitham]

Summary

Replace @"" verbatim strings with """...""" raw string literals (C# 11+) for multi-line content like GraphQL schemas and JSON fixtures in tests.

Motivation

• Raw string literals eliminate the need to escape quotes, making GraphQL schema strings and JSON much more readable
• Particularly beneficial in test files where GraphQL queries and expected JSON results are embedded inline
• We already target C# latest, so this syntax is available

Tasks

• Identify multi-line @"" strings that would benefit (especially in test projects)
• Convert to """...""" raw string literals
• Prioritize test fixtures with embedded GraphQL or JSON