Dedupe outbox type resolution and RabbitMQ wire-message construction

[Vulthil]

Summary

• Extract the RabbitMQ publish/send/request paths' common wire-construction steps (id
  resolution, common BasicProperties fields, envelope serialization, producer activity
  tags) into one shared internal builder used only within Vulthil.Messaging.RabbitMq;
  the fields and error-handling scope that legitimately differ per operation (reply-to,
  wire correlation id, persistence, expiration, serialization failure handling) stay in
  each caller.
• Within the RabbitMQ transport's message-context factory, share the AMQP
  property/header extraction between the fault-snapshot and bare-JSON typed-context
  paths.
• Drop an unused generic type parameter from an internal publish method.
• Document and pin the "last registered transport wins" contract the test harness's
  transport replacement depends on.

Verification

• Full solution build: clean.
• Vulthil.Messaging.Tests, Vulthil.Messaging.RabbitMq.Tests, Vulthil.Messaging.Outbox.Tests,
  Vulthil.Messaging.TestHarness.Tests, Vulthil.Messaging.Abstractions.Tests: all green on
  net9.0 and net10.0, assertions unmodified.
• dotnet pack (Release, fresh obj/bin) for every touched package: clean, no
  API-compatibility errors against the 1.1.0 baseline.
• No InternalsVisibleTo between shipped assemblies: cross-assembly dedup from the first
  revision (outbox type resolver, typed-context mapping) was reverted per review; all
  remaining sharing is within a single assembly.

Backport to v1.0: no — internal refactor ships with the next release.