feat: add request validation

src/request.validation/package-readme.md

@@ -0,0 +1,85 @@
+Request.Validation is a lightweight validation library for C#, built around composable validators, fluent rules, and
+structured validation results.
+
+## Features
+
+- **Composable validators:** Build validators by inheriting from `Validator<T>` and defining rules with `RuleFor` and
+  `RuleForEach`.
+- **Built-in and custom rules:** Use helpers like `NotEmpty`, `Length`, `Between`, and `Matches`, or define custom
+  predicates with `Must`.
+- **Structured validation output:** Each failure is returned as a `Problem` with a property path, message, severity,
+  code, and attempted value.
+- **Nested validation:** Reuse validators for complex object graphs with `SetValidator`, including DI-based resolution.
+
+## Getting Started
+
+### Install the NuGet package:
+
+```shell
+dotnet add package Geekeey.Request.Validation
+```
+
+You may need to add our NuGet feed to your nuget.config this can be done by running the following command:
+
+```shell
+dotnet nuget add source -n geekeey https://code.geekeey.de/api/packages/geekeey/nuget/index.json
+```
+
+### Usage
+
+```csharp
+using Geekeey.Request.Validation;
+
+public sealed record Address(string? Street);
+
+public sealed record CreateUserRequest(
+  string? Name,
+  int Age,
+  Address? Address,
+  IReadOnlyList<string> Tags);
+
+public sealed class AddressValidator : Validator<Address>
+{
+  public AddressValidator()
+  {
+    RuleFor(address => address.Street)
+      .NotEmpty();
+  }
+}
+
+public sealed class CreateUserRequestValidator : Validator<CreateUserRequest>
+{
+  public CreateUserRequestValidator()
+  {
+    RuleFor(request => request.Name)
+      .NotEmpty()
+      .Length(2, 100)
+      .WithCode("NAME_INVALID");
+
+    RuleFor(request => request.Age) 
+      .Between(18, 120);
+
+    RuleFor(request => request.Address)
+      .SetValidator(new AddressValidator());
+
+    RuleForEach(request => request.Tags)
+      .NotEmpty()
+      .WithSeverity(Severity.Warning);
+  }
+}
+
+var validator = new CreateUserRequestValidator();
+var validation = validator.Validate(new CreateUserRequest(
+  Name: "",
+  Age: 16,
+  Address: new Address(""),
+  Tags: ["", "admin"]));
+
+foreach (var problem in validation.Problems)
+{
+  Console.WriteLine($"{problem.PropertyName}: {problem.Message}");
+}
+```
+
+The resulting `Problem` entries include full property paths like `Address.Street` and `Tags[0]`, making it easy to
+surface validation errors back to callers or APIs.