Fix ValidationReporter: remove static, wire via DI

[Copilot]

ValidationReporter was declared static but registered as a DI singleton, causing a build failure (CS0718: static types cannot be used as type arguments).

Changes

• ValidationReporter.cs: Remove static from class and method declarations
• ValidatorRunner.cs: Inject ValidationReporter via constructor; call as instance method

// Before
internal static class ValidationReporter { ... }
await ValidationReporter.Report(results); // called statically

// After
internal class ValidationReporter { ... }
internal sealed class ValidatorRunner(SlnxFileResolver resolver, ValidationCollector collector, ValidationReporter reporter)
{
    ...
    await reporter.Report(results); // injected instance
}

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