Converts all 73 test source files from file-scoped (namespace X;) to
block-scoped (namespace X { ... }) declarations, re-indenting their
contents accordingly. Matches the existing convention already used in
several production files (e.g. Musify.Infrastructure.Configuration,
Musify.Application.Contracts).
Mechanical change only, done with a script, verified with a full
rebuild (0 warnings, 0 errors) and test run (257/257 passing).