fix(enums): skip nested StringEnum generation

[kathelon]

Summary

• skip [StringEnum] generation for nested enums by requiring top-level enum symbols in the incremental pipeline
• add a regression test that keeps a nested public enum in the test assembly and verifies no generated extensions type is emitted
• update the README test badge count to match the added test

Why

The generator was only filtering out private enums. A nested public enum inside a type could still enter the pipeline even though the implementation assumes top-level enums when building generated type names.

Validation

• dotnet test CSharpEssentials.Tests/CSharpEssentials.Tests.csproj --filter StringEnumGeneratorTests
• dotnet build

Fixes #5.

[senrecep]

Fix looks correct — ContainingType is null covers all nesting levels.

Two small things:

Test name — Nested_Enum_Should_Not_Generate_Extensions doesn't say what's being tested. Other tests in the project follow MethodName_Should_X_When_Y, this one drifts from that.

Edge case — only class-nested is covered. A struct-nested test would be nice to confirm the same behavior holds there too.

Neither is a blocker, worth a look while the PR is open.