Allow removing notification sources from the DnD picker

[maruthang]

Summary

Adds a per-row "Remove Source" trash button to the "Notifications: Toggle Do Not Disturb Mode By Source..." quick-pick (and the status-bar "Configure Do Not Disturb..." entry, which opens the same picker), implementing the action @bpasero suggested in #248913.

Fixes #248913

Implementation

• A new removeSourceButton: IQuickInputButton is added to every quick-pick item, using Codicon.removeClose, a localized "Remove Source" tooltip, and alwaysVisible: true.
• A picker.onDidTriggerItemButton handler:
  1. Calls the existing INotificationService.removeFilter(item.id) API (no new service surface — removeFilter is already wired through to storage and is what extension-uninstall flows already use internally).
  2. Rebuilds picker.items minus the removed entry.
  3. Preserves the user's existing checkbox selection for the surviving rows by id.
• The existing onDidAccept handler is unchanged. Because it iterates picker.items, the removed source is not re-added on accept — it simply disappears from the list. This matches the user's expectation that "Remove" means "forget this source entirely."
• Removal is triggered by explicit user action only; existing automatic behavior (sources being re-added when an extension that posts a notification reappears) is unchanged.

Test plan

• New unit tests in notificationService.test.ts cover the removeFilter/getFilters API the picker depends on:
  • getFilters returns sources registered via setFilter.
  • removeFilter drops a tracked source and reverts its per-source filter to the default.
  • removeFilter is a no-op for unknown ids.
  • Removal persists across NotificationService instances backed by the same storage.
  • Removing the last filter leaves the source list empty.
• Manual: install an extension that posts a notification, run "Notifications: Toggle Do Not Disturb Mode By Source...", click the trash icon on the row — row disappears, picker stays open with surviving rows selectable.
• Manual: uninstall the extension, re-open the picker — the source no longer appears in the list (even though it was previously stored).
• Manual: removing the last source leaves the picker empty but accepts cleanly.

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