Fix multimodal serialization

[sidnarayanan]

1. We were losing content_is_json_str
2. Downstream code would sometimes incorrectly check Message.content, which can sometimes be a serialized list. As a helper, we provide Message.serialized_content. Should we cache this? Up for discussion.

[Copilot]

Pull request overview

Fixes multimodal Message serialization so content_is_json_str is not lost in certain dump/roundtrip flows, and provides a safer helper for downstream code to use API-ready content.

Changes:

• Ensure content_is_json_str is emitted during model_dump(..., context={"include_info": True}) when applicable.
• Centralize content shaping during serialization via a new Message.serialized_content helper.
• Add/extend tests covering multimodal dump expectations and dump→validate roundtrips.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/aviary/message.py	Adjusts serialization logic and preserves content_is_json_str when include_info is set; adds serialized_content helper.
tests/test_messages.py	Adds expectations and roundtrip tests for multimodal serialization and content_is_json_str preservation.

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[jamesbraza]

Nice work! Tests are great