fix: preserve binary response body as Buffer in transformFCResponse

[Blankll]

Problem

When @geek-fun/serverless-adapter returns a response with isBase64Encoded: true for binary content types (e.g. image/png), transformFCResponse was converting the base64-decoded data to a UTF-8 string via Buffer.from(body, 'base64').toString('utf-8').

This corrupted binary data — bytes >= 0x80 became U+FFFD (0xEF 0xBF 0xBD). The result was then passed to respondRaw() which called res.end(string), sending corrupt UTF-8 instead of raw binary bytes.

Fix

Detect binary content types by checking the content-type header:

• image/, audio/, video/, application/octet-stream, application/pdf, font/ → keep decoded body as Buffer
• All other types (text, JSON, etc.) → convert to UTF-8 string as before

This allows respondRaw() to call res.end(buffer) with raw bytes for binary content, while preserving backward compatibility for text/JSON responses.

Changed Files

• src/stack/localStack/aliyunFc.ts: Binary content-type detection + Buffer preservation
• tests/unit/stack/localStack/aliyunFc.unit.test.ts: Updated PNG test to verify Buffer output with Buffer.equals() byte equality check

Depends On

• fix: preserve binary response bodies through adapter chain serverless-adapter#16 (binary response base64 encoding)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.42%. Comparing base (f11e2a1) to head (106d50a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   88.38%   88.42%   +0.03%     
==========================================
  Files         145      145              
  Lines        7639     7643       +4     
  Branches     1998     2001       +3     
==========================================
+ Hits         6752     6758       +6     
  Misses        397      397              
+ Partials      490      488       -2     

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