Preserve literal plus signs when rendering mailto text

[OskarEichler]

Summary

Decode mailto components using URI percent decoding, preserving literal plus signs. Form decoding incorrectly treats + as a space in recipients, header names/values and body.

Reproduction

URI('mailto:a+b@example.test?subject=a+b&body=x+y').to_mailtext currently renders a space in each plus position. Afterward it preserves a+b and x+y. %2B still decodes to plus and %20 to space. RFC 6068 section 2 permits literal plus in mailto components.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 93 tests / 3,039 assertions / zero failures or errors, both baseline and this isolated patch.
• 6 full-output checks cover literal plus, encoded plus and encoded spaces across the recipient, additional to field, subject, custom header and body, including the to_rfc822text alias.
• Syntax and git diff --check pass. Supplemental Lint adds no findings against the 31-finding baseline.
• No test/spec files added or modified, per this contribution's explicit no-new-tests constraint. Focused reproductions ran externally.

Compatibility and limitations

Intentional correction: literal plus no longer becomes a space. This does not change header-case handling or broader mailto parsing. No public API removal, dependency change or Ruby minimum change.

Based on master 696df43b0be4c05d3a0dcf7db582126c915d860d. Other Ruby/OS runtimes were not executed locally. No production traffic or live mail/FTP services were used. Existing related PRs/issues were checked; this is a focused contribution, not exhaustive behavioral coverage.