fix: preserve fetch multipart body on MockAgent fallback

[mcollina]

Summary

• preserve fetch's serialized request body when a MockAgent falls back to net connect
• keep multipart/form-data boundaries consistent between the Content-Type header and request body
• add a regression test for issue Incorrect multipart/form-data boundary in header/body with MockAgent #5241

Testing

• npx borp -p "test/fetch/issue-5241.js"
• npx borp -p "test/mock-agent.js"
• npm run lint -- lib/web/fetch/index.js lib/mock/mock-utils.js test/fetch/issue-5241.js

Fixes #5241

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (e58fff7) to head (07f4094).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5269   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         110      110           
  Lines       36353    36384   +31     
=======================================
+ Hits        33909    33938   +29     
- Misses       2444     2446    +2     

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