fix: IndexError in detect_media_type_from_base64 for non-base64 data …

[Serjbory]

fix: IndexError in detect_media_type_from_base64 for non-base64 data URIs

Motivation and Context

detect_media_type_from_base64(data_uri=...) raises an unhandled IndexError when the provided URI does not contain the ;base64, separator (e.g. data:text/plain,Hello%20World or a plain HTTP URL). The function blindly calls data_uri.split(";base64,", 1)[1] without first checking that the delimiter exists.

The sister function _get_data_bytes_as_str in the same file already guards against this case, but detect_media_type_from_base64 was missing the same check.

Description

• Added a guard in detect_media_type_from_base64 that raises a ValueError("Data URI must use base64 encoding.") when data_uri does not contain ;base64,, consistent with the existing pattern in _get_data_bytes_as_str.
• Added two test assertions to test_data_content_detect_image_format_from_base64 covering a non-base64 data URI (data:text/plain,Hello%20World) and a plain HTTP URL (http://example.com/image.png).

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• [no] Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Serjbory]

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