Fix data URI TryParse contract

[fallintoplace]

Summary

• catch invalid base64 payloads in DataEncodingHelpers.TryParseDataUri and return false instead of leaking FormatException
• add a chat smoke test that deserializes an invalid file data URI and verifies it is rejected with ArgumentException

Notes

• this keeps the existing file-content validation behavior intact while fixing the Try* helper contract underneath it
• I intentionally did not change the separate image_url fallback behavior in this PR
• I was not able to run the test suite locally because the dotnet SDK is not installed in this environment