Mail Activity Cancel Tracking

This module adds a trace in the chatter whenever a scheduled activity is cancelled or deleted, so the history of a record no longer loses that information.

• When an activity is removed (for example by deleting it from the activity list or the record), a message is automatically posted on the related record.
• The message shows the activity type name and icon, the word "canceled", and, if the activity was assigned to someone other than the person doing the cancellation, the name of the original assignee.
• If the activity had a summary, it is included after a colon; if it had a note, the note is shown under an "Original note" heading.
• The message is posted under the "Activities" subtype (mail.mt_activities), so it appears in the chatter like other activity-related messages.
• Marking an activity as done does NOT create this cancellation message: the module explicitly skips the log in that case, so only real cancellations are tracked.
• The change applies to the mail.activity model, which is the model behind all scheduled activities in Odoo, so it works for any record that supports activities (sales orders, contacts, projects, etc.).

No new menus or screens are added; the only visible change is the extra message in existing chatter threads.