[WIP] Fix DataContent mapping to embedded resources

[Copilot]

• Understand the current code: DataContent catch-all at line 412 always creates BlobResourceContents for embedded resources, even when the MIME type is text-based
• Add a new case in the ToContentBlock switch expression: when DataContent has a top-level media type of "text", map it to EmbeddedResourceBlock with TextResourceContents instead of BlobResourceContents
• Update existing tests that assume text MIME types round-trip as BlobResourceContents
• Add a new test for text MIME type DataContent → TextResourceContents round-trip
• Build and run tests to verify the fix
• Code review and security scan

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