Fix UTF-8 encoding for foreign characters in ChatGPT responses

[Copilot]

Problem

Foreign characters like "è" were being displayed as "Ã" in ChatGPT API responses. This occurred because the VBA code was using responseText which doesn't properly handle UTF-8 encoding from the ChatGPT API.

Root Cause

The ChatGPT API returns responses in UTF-8 encoding, but VBA's responseText property treats the response as Windows-1252/Latin-1. Characters like "è" (UTF-8: 0xC3 0xA8) were being interpreted as two separate bytes, with 0xC3 displaying as "Ã" in Windows-1252.

Solution

Added proper UTF-8 decoding by:

1. New UTF8ToVBAString() function - Uses ADODB.Stream to convert UTF-8 byte arrays to proper VBA strings
2. Modified GetChatGPTResponse() - Now uses httpRequest.responseBody instead of responseText
3. Enhanced error handling - Fallback mechanism if ADODB.Stream is unavailable
4. Code quality improvements - Added missing variable declarations

Example

Before: Response containing "café" would display as "café" After: Response properly displays as "café"

Technical Details

• Uses ADODB.Stream with UTF-8 charset to properly decode response bytes
• Maintains backward compatibility with existing functionality
• Minimal changes (31 lines added, 1 line modified)
• Robust error handling ensures the function works even if ADODB fails

Fixes #2.

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