fix: preserve utf8 decoding across response chunks

[pupuking723]

This relates to...

Fixes #5002

Rationale

BodyReadable#setEncoding() only stored the encoding on the readable state. That made streamed consumers decode each buffered chunk independently, so an incomplete UTF-8 sequence at a response chunk boundary produced replacement characters.

The internal body consumers still need raw Buffer chunks so body.text() and body.json() can aggregate and decode the full payload.

Changes

Features

N/A

Bug Fixes

• Keep raw Buffer chunks for internal body consumption.
• Use a StringDecoder when BodyReadable is consumed through the Readable API after setEncoding().
• Add a regression test for async iteration over a response body where a 3-byte UTF-8 character spans response chunks.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Verification:

• node --test --test-name-pattern "request multibyte (json|text) with setEncoding|async iteration and setEncoding" test/client-request.js
• npx borp --timeout 180000 -p test/client-request.js
• npm run lint
• npm run test:typescript
• git diff --check

Note: npm ci --ignore-scripts was used locally because npm install exited with "Exit handler never called!" after dependency extraction. The test/lint commands above were run against the installed dependency tree. Local Node.js is v22.17.0 while the package currently requires >=22.19.0.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.78261% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.24%. Comparing base (ac5394b) to head (85fc024).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/api/readable.js	84.78%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5414      +/-   ##
==========================================
- Coverage   93.25%   93.24%   -0.02%     
==========================================
  Files         110      110              
  Lines       36825    36868      +43     
==========================================
+ Hits        34340    34376      +36     
- Misses       2485     2492       +7     

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[mcollina]

I'm relatively certain that Readable implements this logic. I think the problem is in the override of setEncoding, we should also call super.setEncoding