fix(context): clean multipart temps after ServeHTTP

[RubenPari]

Summary

• Call unexported Context.cleanupMultipartForm() at the end of Engine.ServeHTTP so temp files from ParseMultipartForm are removed even when middleware replaces c.Request (e.g. OTEL WithContext).
• net/http only runs MultipartForm.RemoveAll() on the original request handed to ServeHTTP; a shallow-copied request leaves /tmp/multipart-* orphaned (Why no call to Request.MultipartForm.RemoveAll #4278).
• Double RemoveAll is safe when the request was not replaced (os.ErrNotExist ignored). No public API change. Supersedes the approach in clean up the multipart request files- this resolves #4278 #4279 (cleanup must live in Gin's request lifecycle, not binding).

Test plan

• go test ./... -count=1 -run 'TestServeHTTPCleansMultipartForm|TestContextCleanupMultipartFormNilSafe'
• go test . -count=1
• Regression: middleware replaces request via WithContext, FormFile with body > MaxMultipartMemory → temp path gone after ServeHTTP
• Same without middleware replace (double-cleanup path)
• cleanupMultipartForm nil-safe for nil Request / nil MultipartForm

Fixes #4278

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.65%. Comparing base (3b08cd7) to head (ed90302).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4791   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          48       48           
  Lines        3190     3194    +4     
=======================================
+ Hits         3147     3151    +4     
  Misses         43       43           

Flag	Coverage Δ
--ldflags="-checklinkname=0" -tags sonic	98.64% <100.00%> (+<0.01%)	⬆️
-tags go_json	98.58% <100.00%> (+<0.01%)	⬆️
-tags nomsgpack	98.63% <100.00%> (+<0.01%)	⬆️
go-1.26	98.34% <100.00%> (+<0.01%)	⬆️
go-1.27	98.67% <100.00%> (+<0.01%)	⬆️
macos-latest	98.65% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	98.65% <100.00%> (+<0.01%)	⬆️

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