Fix AES key charset encoding issue in WxPay decryptToString method

[Copilot]

The WeChat Pay V3 API notification parsing was failing with "Invalid AES key length: 33 bytes" error due to inconsistent charset encoding when converting strings to byte arrays.

Problem

The AesUtils.decryptToString() method was using String.getBytes() without specifying the charset, which caused platform-dependent behavior:

// Problematic code - charset dependent
SecretKeySpec key = new SecretKeySpec(apiV3Key.getBytes(), "AES");
GCMParameterSpec spec = new GCMParameterSpec(TAG_LENGTH_BIT, nonce.getBytes());
cipher.updateAAD(associatedData.getBytes());

When the platform default charset differed from UTF-8, or when the API key contained non-ASCII characters, the byte array length could exceed the required 32 bytes for AES-256 encryption, causing the InvalidKeyException: Invalid AES key length: 33 bytes error.

Solution

Updated all getBytes() calls in the decryptToString() method to explicitly use UTF-8 encoding:

// Fixed code - consistent UTF-8 encoding
SecretKeySpec key = new SecretKeySpec(apiV3Key.getBytes(StandardCharsets.UTF_8), "AES");
GCMParameterSpec spec = new GCMParameterSpec(TAG_LENGTH_BIT, nonce.getBytes(StandardCharsets.UTF_8));
cipher.updateAAD(associatedData.getBytes(StandardCharsets.UTF_8));

This ensures consistent behavior across all platforms and environments, matching the pattern already used in the HMACSHA256() method within the same class.

Impact

• Fixes the "解析报文异常" (parse message exception) error in WeChat Pay V3 notification handling
• Ensures predictable behavior regardless of platform default charset
• Maintains backward compatibility with existing valid configurations
• Follows consistent coding patterns within the codebase

Fixes #3698.

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