fix(core): support string data in Blob.as_bytes_io

[zxuhan7 (zxuhan)]

Closes #36667

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Blob.as_string and Blob.as_bytes both handle data: str (encoding it via self.encoding when bytes are needed), but Blob.as_bytes_io only handled bytes and file paths and raised NotImplementedError for string data. The fix adds the missing isinstance(self.data, str) branch, encoding with the blob's configured encoding for parity with as_bytes.

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