fix: stop adding topic and retain when writing an MQTT lastWill

[bielu]

Problem

Reading an MQTT server binding with a partial lastWill and writing it back changes the document:

# input
bindings:
  mqtt:
    lastWill:
      qos: 1

# output
bindings:
  mqtt:
    lastWill:
      topic:
      qos: 1
      retain: false

LastWill.Serialize wrote topic with WriteRequiredProperty (so a missing topic became topic: null) and retain from a non-nullable bool (so a missing value became retain: false). Both fields are optional in the MQTT binding spec.

Change

• retain now has a private bool? backing field; the public Retain property is still bool (defaults to false), the same pattern MQTTOperationBinding already uses for qos/retain. No public API change.
• topic and retain are written with WriteOptionalProperty, so they appear only when set. An explicitly assigned Retain = false is still written.

Tests

Added to MQTTBindingVariants_Should:

• LastWill_WithoutTopicAndRetain_RoundTripsWithoutAddingThem (V2 and V3): fails on vnext, passes with this change.
• LastWill_WithExplicitRetainFalse_WritesRetain: guards that an explicit false is still written.

The existing V2_MQTTServerBinding_FilledObject_SerializesAndDeserializes round-trip is unchanged, and the full suite passes on net8.0/net9.0/net10.0.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between eb68984 and fa8bb27.

📒 Files selected for processing: 2

• src/ByteBard.AsyncAPI.Bindings/MQTT/LastWill.cs
• test/ByteBard.AsyncAPI.Tests/Bindings/MQTT/MQTTBindingVariants_Should.cs

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