Fix UTF-8 character display in debug page response bodies

[Copilot]

Polish letters (Ł, Ń, ą, etc.) and other UTF-8 characters were rendering incorrectly in the debug page when viewing HTTP response bodies.

Changes

• LoggingHttpClient: Replace http.Response.bytes().body with explicit utf8.decode(responseBodyBytes, allowMalformed: true) to ensure consistent UTF-8 decoding regardless of Content-Type header charset
• Test coverage: Add test case verifying Polish character decoding from byte streams

Technical Detail

Previous implementation relied on the http package's charset detection from Content-Type headers, which could fail or use incorrect encoding. Direct UTF-8 decoding ensures special characters display correctly.

// Before
responseBodyCompleter.complete(
  http.Response.bytes(responseBodyBytes, response.statusCode).body,
);

// After
final responseBody = utf8.decode(responseBodyBytes, allowMalformed: true);
responseBodyCompleter.complete(responseBody);

Original prompt

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This section details on the original issue you should resolve

<issue_title>[debug_page] Polish letters are not displayed</issue_title>
<issue_description> Letters like Ł,Ń,ą etc are not displayed properly</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [debug_page] Polish letters are not displayed #491

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