Map JSON metadata to Kafka message headers, add unit tests

[bukk530]

Why

Implements the TODO to propagate event metadata to Kafka headers when the event payload is JSON, enabling downstream consumers to access metadata without parsing the value body.

What’s changed

• KafkaWriter
  • Build Kafka message headers from metadata only when ContentTypes.Json and metadata is present.
  • Delegates to KafkaHeadersBuilder.BuildHeaders.
  • No behavior change for non‑JSON content.
• KafkaHeadersBuilder
  • Parses metadata JSON and maps only top‑level properties to headers.
  • Header value encoding:
    • String → raw UTF‑8 bytes of the string
    • Non‑string JSON → UTF‑8 bytes of compact GetRawText()
  • Safely ignores malformed JSON, non‑object roots, or empty objects; logs a warning on malformed JSON.
• Tests (NUnit)
  • KafkaHeaderBuilderTests: verifies mapping rules and edge cases.
  • KafkaWriterHeadersTests: integration‑style test with a fake IProducer<string, byte[]> to assert headers on produced messages.
  • Tests use Assert.That syntax.
  • Added CapturingProducer test double implementing required IProducer members.
  • Initialize metrics in [OneTimeSetUp] via ReplicationMetrics.Configure(Metrics.CreateUsing()); to avoid NREs during Metrics.Measure.

Notes

• Closes: TODO “Map meta to headers, but only for JSON”.
• No breaking changes; non‑JSON events remain unaffected.
• I've introduced NUnit for the new tests.
• This PR was created with the help of automated software development methods (AI).

How to test

• Run: dotnet test test/Kurrent.Replicator.Kafka.Tests/Kurrent.Replicator.Kafka.Tests.csproj
• Expected: all tests pass (header mapping for JSON; ignored otherwise).

Manual testing

• The feature was manually tested as per below screenshots

Before:

After:

The manual test was run with the following transform:

function transform(original) {
    const originalMeta = original.Meta || {};

    const meta = {
        ...originalMeta,
        stream: original.Stream,
        eventType: original.EventType,
        createdAt: original.Created,
        replicatedAt: new Date().toISOString()
    };

    return {
        Stream: original.Stream,
        EventType: original.EventType,
        Data: original.Data,
        Meta: meta,
    };
}

[CLAassistant]

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