fix(process-executor): model commands as a CommandLine enum

Commands were carried as `PhpMixed`, whose `String`/`List` variants do
not tell a shell command line apart from an argv list at the type level.
`Perforce::execute_command` and `Git::run_command` therefore funnelled
string commands through `execute_args`, spawning `p4 set` or
`git command` as a single argument instead of running it through a shell
as PHP does; their tests were written against that shape.

Introduce `CommandLine::{Shell, Args}` and use it for every
`ProcessExecutor` entry point, the mock expectation queue and the
`Git::run_command` callables. The unreachable "Invalid command type"
branches disappear with it, and the affected tests go back to the string
expectations the PHP suite uses.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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