fix(http): harden requests and resource cleanup

[ekalinin]

Summary

• inject one HTTP client with a finite timeout into remote GET and POST adapters
• accept only 2xx responses and return typed status errors with bounded body fragments
• limit successful response bodies and validate response media types
• stream POST bodies from files instead of buffering them in memory
• close and remove remote Markdown and stdin temporary files on success and failure
• write debug JSON directly to a single retained artifact

Checks

• go test -count=1 ./...
• go test -race -count=1 ./...
• GOTOOLCHAIN=go1.21.13 go test ./...
• go vet ./...
• golangci-lint run
• go mod tidy -diff

[codecov]

Codecov Report

❌ Patch coverage is 69.92481% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.31%. Comparing base (4d9bf98) to head (7c0bf07).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/utils/utils.go	60.97%	11 Missing and 5 partials ⚠️
internal/controller/stdin.go	41.17%	6 Missing and 4 partials ⚠️
internal/core/usecase/remotehtml/remotehtml.go	78.57%	3 Missing and 3 partials ⚠️
internal/core/usecase/remotemd/remotemd.go	75.00%	3 Missing and 3 partials ⚠️
internal/core/usecase/new.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   79.13%   79.31%   +0.18%     
==========================================
  Files          27       28       +1     
  Lines         623      701      +78     
==========================================
+ Hits          493      556      +63     
- Misses        100      101       +1     
- Partials       30       44      +14     

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