fix(streaming): ensure process_stream returns non-empty message content

[Ratish1]

Description

This PR fixes a bug where AgentResult would contain an empty message list if the model returned a stream with no content events as noticed with Claude 3 Haiku.

Changes:

• Added a check at the end of process_stream in streaming.py to add {"text": ""} if the accumulated content is empty.
• Updated test_streaming.py to assert that empty streams resolve to a message with empty text, rather than an empty list.

Related Issues

Fixes #1264

Documentation PR

N/A

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

• [x] I ran hatch run prepare

Checklist

• [x] I have read the CONTRIBUTING document
• [x] 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
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published

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