feat: KafkaTransportOptions for configurable Kafka outbox transport

[Copilot]

Kafka was the only Pulse transport without a dedicated options class — topic creation used hard-coded partition count and replication factor with no way to configure retention or disable auto-creation.

Changes

New: KafkaTransportOptions

public sealed class KafkaTransportOptions
{
    public int DefaultPartitionCount { get; set; } = 1;
    public short DefaultReplicationFactor { get; set; } = 1;
    public bool AutoCreateTopics { get; set; } = true;
    public TimeSpan? MessageRetention { get; set; }
}

KafkaMessageTransport

• Injects IOptions<KafkaTransportOptions>
• Adds EnsureTopicAsync(): creates the topic before first produce to a given topic name, cached via ConcurrentDictionary to avoid redundant admin calls
• Skips topic creation entirely when AutoCreateTopics = false
• Applies retention.ms to topic config when MessageRetention is set
• Handles TopicAlreadyExists errors gracefully

UseKafkaTransport() extension

• Accepts optional Action<KafkaTransportOptions>? configureOptions = null
• Registers IOptions<KafkaTransportOptions> via AddOptions/Configure
• Fully backward-compatible — omitting the delegate preserves existing default behavior

services.AddPulse(config =>
    config.UseKafkaTransport(opt =>
    {
        opt.DefaultPartitionCount = 3;
        opt.DefaultReplicationFactor = 2;
        opt.MessageRetention = TimeSpan.FromDays(7);
    })
);

Tests

• FakeAdminClient updated to track CreateTopicsAsync calls (CreatedTopics, CreateTopicsCallCount)
• New tests covering: auto-create enabled/disabled, deduplication of topic creation, custom partition count, custom replication factor, custom retention, no retention config when unset, UseKafkaTransport with and without options delegate, constructor null guard

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.19%. Comparing base (890cdb6) to head (b9ad6cb).

Files with missing lines	Patch %	Lines
...Evolve.Pulse.Kafka/Outbox/KafkaMessageTransport.cs	90.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
+ Coverage   92.13%   92.19%   +0.06%     
==========================================
  Files         151      152       +1     
  Lines        5591     5626      +35     
  Branches      524      529       +5     
==========================================
+ Hits         5151     5187      +36     
+ Misses        292      291       -1     
  Partials      148      148              

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