fix(data_connect): fix UTF 8 characters decoding in data connect

[Lyokone]

Description

RestTransport.invokeOperation used http.Response.body to read the server response, which decodes bytes using the charset from the Content-Type header. When the server omits charset=utf-8 (as the Firebase emulator does), the http package defaults to latin1 — corrupting multi-byte UTF-8 characters like Korean, Chinese, Japanese, etc.

Switched to utf8.decode(response.bodyBytes) for both the success and error paths so international characters are always decoded correctly.

Related Issues

• Fixes [firebase_data_connect] Response data Correct UTF-8 decoding for international characters #17290

Checklist

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• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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