Refactor multipart boundary handling in message rendering #189

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	2m 23s
golangci-lint (1.23)	29s
Go vulnerabilities check	17s
Dependency review	13s
REUSE Compliance Check	18s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 21s
Test on FreeBSD 14.0	2m 35s
Test on FreeBSD 13.4'	2m 19s
Test on FreeBSD 14.1	3m 31s
Test (macos-latest / 1.19)	34s
Test (macos-latest / 1.20)	29s
Test (macos-latest / 1.21)	23s
Test (macos-latest / 1.22)	22s
Test (macos-latest / 1.23)	23s
Test (ubuntu-latest / 1.19)	30s
Test (ubuntu-latest / 1.20)	27s
Test (ubuntu-latest / 1.21)	28s
Test (ubuntu-latest / 1.22)	21s
Test (ubuntu-latest / 1.23)	26s
Test (windows-latest / 1.19)	1m 4s
Test (windows-latest / 1.20)	2m 32s
Test (windows-latest / 1.21)	2m 7s
Test (windows-latest / 1.22)	2m 47s
Test (windows-latest / 1.23)	2m 6s
	28m 25s

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