fix: use enums instead of string literals for message actions in java and python

[ttypic]

Description

• Use enums instead of string literals for message actions in java and python
• Use messages constructor for python publishes

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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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 AIT-278/small-fix

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[GregHolmes]

Suggested change

	elif action == MessageAction.MESSAGE_UPDATE':
	elif action == MessageAction.MESSAGE_UPDATE:

[GregHolmes]

Suggested change

	elif action == MessageAction.MESSAGE_APPEND':
	elif action == MessageAction.MESSAGE_APPEND: