Thin veraPDF commands via shared install/env logic

[jbagsik]

Context

SonarQube duplication on the moox/verapdf branch in production code: InstallVeraPdfCommand (12%), ValidateCommand (26%), and DoctorCommand repeat the same concerns — resolving base path / launcher / Java binary, the "installed / java-available" gating, and status output.

What to build

The veraPDF commands share their install/environment logic through one place instead of repeating it, so each command is thin and the shared logic has a single home.

Acceptance criteria

• Shared path/Java/installed-gating/status logic is extracted into a trait or VeraPdfService and used by InstallVeraPdfCommand, ValidateCommand, and DoctorCommand.
• Command behavior and output are unchanged (existing command tests still pass).
• SonarQube duplication on the command files drops.

Blocked by

None — can start immediately.