Harden core template behavior

[codefoxx]

Summary

Hardens reusable core template behavior with focused contract tests around application results, validation, API error mapping, global exception handling, feature composition, and generated-template validation.

Why

The Products feature is only a sample proof slice. Core template behavior such as Result/Error/Option semantics, validation aggregation, HTTP ProblemDetails mapping, exception handling, and feature composition should be protected directly so the template remains safe to evolve and easier to trust.

Changes

• Adds direct contract coverage for Result, Result<T>, Option<T>, Error, and ErrorCode.
• Adds validation and ValidationResult<T> behavior tests.
• Adds direct tests for EndpointResultExtensions and HTTP ProblemDetails mapping.
• Adds direct tests for GlobalExceptionHandler safe unexpected-failure behavior.
• Extends Feature Composition tests for configuration, ordering, decorator behavior, and guard clauses.
• Splits large core contract test classes into nested partial files for IDE readability.
• Aligns tests with explicit // Arrange, // Act, // Assert sections.
• Keeps tests focused on reusable template behavior instead of expanding the Products sample domain.

Design notes

• No new production dependencies.
• No broad architecture rewrite.
• No deep Products business testing.
• No test project reshuffle; tests stay in the current layer-based projects for now.
• Partial contract test files avoid template source-variant suffixes such as .None.cs, .PostgreSql.cs, and .SqlServer.cs.

Validation

dotnet test content/Company.Template.slnx
./scripts/template.sh all-postgres