fix(weixin): check full 4-byte PNG magic signature

[chinesepowered]

Check full 4-byte PNG magic signature in Weixin image MIME detection.

TLDR

WeixinAdapter.detectImageMime checked only 3 of the 4 bytes of PNG's magic signature (89 50 4E 47), while the WebP branch in the same function correctly checked all 4 bytes. This is a clear oversight, not a deliberate relaxation. Extend the PNG check to include the fourth byte (0x47, 'G') for consistency.

Screenshots / Video Demo

N/A — byte-level MIME detection has no user-visible surface.

Dive Deeper

Before:

function detectImageMime(data: Buffer): string {
  if (data[0] === 0x89 && data[1] === 0x50 && data[2] === 0x4e) {
    return 'image/png';   // checks only 3 of 4 signature bytes
  }
  if (data[0] === 0x47 && data[1] === 0x49 && data[2] === 0x46) {
    return 'image/gif';
  }
  if (
    data[0] === 0x52 &&
    data[1] === 0x49 &&
    data[2] === 0x46 &&
    data[3] === 0x46
  ) {
    return 'image/webp';  // WebP: full 4-byte RIFF check
  }
  return 'image/jpeg';
}

PNG's magic bytes are 89 50 4E 47. The check only validated the first three. In practice, false positives are nearly impossible — any file starting with \x89PN is essentially always PNG — but the inconsistency with the WebP branch suggests the omission was unintentional rather than a deliberate relaxation.

After:

if (
  data[0] === 0x89 &&
  data[1] === 0x50 &&
  data[2] === 0x4e &&
  data[3] === 0x47
) {
  return 'image/png';
}

Scope note: The GIF check has a similar issue (the full signature is GIF87a or GIF89a, 6 bytes) but a 3-byte GIF prefix is uniquely identifying in practice and is arguably acceptable. Left untouched to keep this PR focused.

Modified file:

• packages/channels/weixin/src/WeixinAdapter.ts — added data[3] === 0x47 check

Reviewer Test Plan

1. Unit-level: construct a Buffer starting with \x89\x50\x4E\x47 → should still return 'image/png'
2. Construct a Buffer starting with \x89\x50\x4E + any other fourth byte (e.g., \x00) → now returns 'image/jpeg' (fallback); previously returned 'image/png'
3. WebP/GIF/JPEG detection unchanged — regression check with real image buffers
4. The function has no existing tests, so no test file changes needed

Testing Matrix

	macOS	Windows	Linux
npm run	?	pass	?
npx	?	?	?
Docker	?	?	?
Podman	?	-	-
Seatbelt	?	-	-