Python: Use the public URI constructor in the Message example - #9332

Motivation & Context

Copying the public Message example raises AttributeError because it calls Content.from_image_uri, which does not exist. The example stops before its serialization demonstrations.

Description & Review Guide

• What are the major changes? Use the existing Content.from_uri constructor with a concrete PNG URL and its media type.
• What is the impact of these changes? The complete documented example runs successfully, including mixed-content construction and both serialization round trips. Only the docstring changes; the runtime AST is unchanged. The actual public example failed before the change and passed on Windows Python 3.11.17 and Linux Python 3.13.16 afterward. Normal scoped repository hooks passed without changing the patch.
• What do you want reviewers to focus on? The example uses the existing public URI constructor and still demonstrates an image alongside text.

Related Issue

N/A - trivial change. This corrects a nonexistent method in a public example; the intended constructor already exists.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: OpenAI Codex assisted with diagnosis, the focused documentation correction, verification and this description. The contributor personally reviewed and verified this change.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.