Cover the command registry, the backup prompt and two grid branches

[blaipr]

What

4 tests on the last small gaps in the CLI and grid plumbing.

• The command registry — CliCommandHelper::getCommands() is the whole sp:* list. An entry dropped from it disappears from the command line with nothing else anywhere to catch it, so it is asserted as the exact list, in order.
• The backup command's interactive path prompt, which had never been driven.
• Two branches of the grid every management screen renders through: updating the pager when there is none attached is a no-op rather than a failure, and the template check's success path — only its failure was covered.

Left alone deliberately

• The backup's not-installed guard sits behind !defined('TEST_ROOT'), which the suite defines for the whole process — structurally unreachable from any test in it.
• The XML export's three catch blocks around DOMDocument cannot be reached through any input: the document is built internally, and invalid text content is sanitised by ext-dom rather than raising. Confirmed experimentally rather than assumed.
• A handful of plain accessors on the grid with no branching, including a $tableTemplate property that has no setter anywhere in the codebase.

One defect found, fixed separately

XmlExport::export() deletes the previous export before writing the new one, so a run that fails part-way destroys the last good export rather than leaving it in place. BackupFile::doBackup() has the same ordering. That is its own change.

Testing

Unit: 13 green across the touched classes. Integration: 6 green on the backup command.