fix(application): typed Result<T>.Failure on validation failures (POM-482)

[Pomdapis]

Summary

Fixes POM-482 — ValidationBehavior was returning null (cast to TResponse) for the Result<T> failure path, silently corrupting the CQRS pipeline for any handler whose response is Result<TValue>.

Root cause

src/Application/Compendium.Application/CQRS/Behaviors/ValidationBehavior.cs:40-44 did :

```csharp
var failureMethod = typeof(Result<>).MakeGenericType(resultType)
.GetMethod(nameof(Result.Failure), new[] { typeof(Error) });

var result = failureMethod?.Invoke(null, new object[] { error }); // ← null in production
return (TResponse)result!;
```

`Failure(Error)` is a static on the non-generic `Result` base class (src/Core/Compendium.Core/Results/Result.cs:88). `GetMethod` on the closed generic `Result` does not surface inherited statics without `BindingFlags.FlattenHierarchy`, so the lookup returned `null` and the null-conditional invocation silently produced `null`.

Fix

Resolve the static through the non-generic `Result` type and close it over the response's inner type via `MakeGenericMethod` :

```csharp
var failureMethod = typeof(Result)
.GetMethods(BindingFlags.Public | BindingFlags.Static)
.Single(m => m.Name == nameof(Result.Failure)
&& m.IsGenericMethodDefinition
&& m.GetParameters().Length == 1
&& m.GetParameters()[0].ParameterType == typeof(Error))
.MakeGenericMethod(resultType);
```

Test

The existing `HandleAsync_WhenInvalidAndResponseIsGenericResult_ShortCircuitsBeforeNext` was renamed to `_ReturnsFailureAndShortCircuits` and now asserts :

• `actual.Should().NotBeNull()`
• `actual.IsFailure.Should().BeTrue()`
• `actual.Error.Code.Should().Be("Validation.Failed")`
• `actual.Error.Type.Should().Be(ErrorType.Validation)`
• `nextCalled.Should().BeFalse()`

The previous version accepted the buggy `null` with a comment — that comment is now removed.

Verification

• `dotnet test tests/Unit/Compendium.Application.Tests` → 225/225 green locally.
• No regression in any other Application test (LoggingBehavior, TransactionBehavior, IdempotencyBehavior, dispatchers, etc.).
• No prod-code change outside the affected method body + a single `using System.Reflection;`.

Release impact

After merge, target framework v1.0.1 → republishes `Compendium.Application`.

Test plan

• CI green (build + 90 % coverage gate).
• CodeQL green.
• User reviews and merges.

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