Add missing code style rules that codify the idioms already used in the
codebase, making IDE suggestions consistent for all contributors:
- csharp_style_prefer_range_operator / prefer_index_operator: prefer
range indexers ([..n], [n..], [^1]) over Substring / Length-based
arithmetic (directly reinforces the refactoring in GatewayUrlHelper)
- csharp_style_prefer_switch_expression: prefer switch expressions
over if/else chains (already used throughout Models.cs etc.)
- csharp_style_prefer_pattern_matching / prefer_not_pattern
- csharp_prefer_simple_using_statement: prefer declaration-style
'using var x = ...' without a nested block
- dotnet_style_prefer_simplified_boolean_expressions
- dotnet_style_prefer_inferred_tuple_names /
prefer_inferred_anonymous_type_member_names
No functional changes. 521 shared + 99 tray tests pass.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
