fix(multiagent): set empty text part data in parts for Artifact

[punkyoon]

Description

The A2A specification states that parts "must contain at least one part."

However, strands-agents emits the final TaskArtifactUpdateEvent without any parts data.

This fix ensures compliance by sending a TextPart with an empty string.

Related Issues

resolve #1640

Documentation PR

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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