PDJB-NONE: Tag the leave-property commit step for the Transaction metric

[Bill-Haigh]

Ticket number

PDJB-NONE

Goal of change

The "no longer a joint landlord" journey was not being counted in the "Total number of transactions" metric on /system-operator/metrics.

Description of main change(s)

• Completing the leave property journey now fires the Plausible Transaction event, so it is counted alongside the other journeys. The commit button was rendering from the untagged warningButton fragment; it now uses transactionWarningButton, matching how property deregistration already does it. Same red destructive button, no visual change.
• Documents two accepted coverage gaps in MetricsReadMe: cancelling a joint landlord invitation, and landlord address updates.
• Adds a tagging rule to the journey instructions so new journeys don't repeat the omission.

Anything you'd like to highlight to the reviewer?

Landlord address updates are deliberately left uncounted. They're the other journey missing the tag, but AddressTask has three routes to its commit and the button fragment is chosen at render time, before the user's radio selection is known — so tagging both select-address and manual-address would fire the event twice whenever someone picks "Add address manually" from the select-address page. Tagging only manual-address would count an arbitrary subset instead. I originally implemented the double-tagged version, then backed it out: under-counting is preferable to double-counting, and the gap is now recorded in MetricsReadMe with the reasoning. Happy to revisit if you'd rather have a JavaScript opt-out keyed on the selected radio.

The same reasoning applies to cancelling a joint landlord invitation, which is structurally identical to the already-documented landlord-deregistration-with-no-properties gap: its only button is on a yes/no are-you-sure page, so tagging it would fire the event even when the user answers "no".

Checklist

• TODO comments referencing this JIRA ticket have been searched for and removed - if a future PR will address them, mention that here
• Test suite has been run in full locally and is passing
• Branch has been rebased onto main and run locally, with everything working as expected (both for your new feature and any related functionality)
• This feature is not behind a feature flag.