fix: Properly escape string litterals and support implict usings false

[nikcio]

• Understand the review feedback (emit using directives conditionally)
• Implement NeedsSystemNamespace() helper using schema-format inspection
• Implement NeedsCollectionsNamespace() helper using schema-type inspection
• Implement NeedsGlobalizationNamespace() helper checking date format + default value
• Add SchemaResolvesToSystemType() and SchemaResolvesToCollectionType() static helpers
• Update Emit() to conditionally emit using directives (restore original flags for System.IO and System.Text.Json)
• All 192 tests pass
• Code review passed - no comments
• CodeQL security scan passed - 0 alerts

[Copilot]

Pull request overview

This PR improves C# code generation correctness for string escaping (notably backslashes) and adds coverage to ensure generated output compiles even when the consuming project disables implicit usings.

Changes:

• Introduces a centralized C# string literal escaping helper and uses it for enum member names, JSON property names, and string defaults.
• Updates generated file headers to include explicit using directives to avoid relying on consuming-project implicit usings.
• Adds new integration-style tests for backslash escaping and for compiling generated code with ImplicitUsings=disable.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/OpenApiCodeGenerator.Tests/CSharpSchemaGeneratorTests.cs	Adds compilation harness helper plus new round-trip tests covering backslash escaping and a compile-only test with implicit usings disabled.
src/OpenApiCodeGenerator/CSharpCodeEmitter.cs	Adds EscapeCSharpStringLiteral and applies it in emitted attributes/defaults; emits explicit using directives in generated output.

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