Clarify test comment about contentEncoding and bidirectional conversion

[Copilot]

The comment in content_media_type_test.dart incorrectly stated "In JSON, the List is UTF-8 decoded to string (not base64)", which contradicted the schema's contentEncoding: base64 declaration and actual server behavior.

Changes:

• Updated comment to acknowledge the schema has contentEncoding: base64
• Clarified that this test uses ASCII-compatible bytes for demonstration purposes
• Explained the test shows bidirectional conversion with simplified data

The schema definition:

imageData:
  type: string
  contentEncoding: base64
  contentMediaType: image/png

The test uses "Hello" (ASCII bytes) as a simple case, not as the general encoding mechanism.

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