feat: introduce MessageContext to simplify passing message identifiers

[pauci]

This PR introduces MessageContext that allows to bundle the three message identifiers (id, causationId, correlationId) into a single value object, reducing method signature complexity from 3 parameters to 1.

The new ContextAwareMessageTrait demonstrates this pattern, and creating child messages is simplified to $context->child() instead of manually propagating identifiers.

This is a non-breaking addition - existing MessageTrait usage remains unchanged.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch message-context

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[pauci]

I also considered naming this class MessageIdentifiers, but I wanted it to be more agnostic, so the messages don't care what it actually contains.