fix(diagnose-command): model check results as dedicated enums

The `check_*` methods returned `PhpMixed`, and `output_result` reproduced
PHP's truthiness over it incorrectly: only `PhpMixed::Bool(false)` took the
falsey path, so an empty string or an empty list printed a blank line
instead of reporting `FAIL` and raising the exit code. `CheckResult` and
`GithubRateLimit` make the shapes PHP returns explicit, and `check_platform`
stores the detail of an error or warning as `Option<String>` rather than
`true`-or-string.

`check_http` and `check_composer_repo` also built the exception label with
`std::any::type_name_of_val`, printing the Rust type path where PHP prints
`get_class($e)`; they now go through `AnyThrowable::php_class_name` like the
rest of the file.

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

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