Python: Make media type detection documentation examples runnable

[BlueRaddish]

Motivation & Context

Both detect_media_type_from_base64 documentation examples use a truncated base64 string containing .... Copying either example raises ValueError: Invalid base64 data provided. instead of returning the documented image/png result.

Description & Review Guide

• What are the major changes? Replace the truncated payloads with the valid base64-encoded PNG signature and describe that signature in the comment.
• What is the impact of these changes? Both examples execute successfully. Runtime behavior and public APIs remain unchanged.
• What do you want reviewers to focus on? The payload represents the PNG signature bytes used by this magic-byte detector, rather than a complete image file. Both examples were extracted from the actual docstring and executed offline against the selected checkout: two ValueError failures before the change, two image/png results after it. Scoped Ruff lint and formatting checks and all normal scoped prek hooks also pass, with the reviewed diff unchanged. Prepared with assistance from OpenAI Codex.

Related Issue

Fixes #9187

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 is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

[Copilot]

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[Eduard van Valkenburg (eavanvalkenburg)]

/review

[github-actions]

MAF Automated Review — Iteration 1

Result: No findings
Scope: full PR (1 commit(s)): d32a23a7afc4
Model: gpt-5.6-sol

Overview

This documentation-only change replaces two invalid truncated Base64 examples with the complete encoding of the PNG signature. The corrected string passes strict Base64 decoding, exercises both the direct-string and data-URI paths, and matches the implementation's PNG magic-byte check without changing runtime or public API behavior. No publishable risk was established.

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
No publishable findings remained after source verification for this scope.