feat: add tooltip field to workspace app that renders as markdown (#19651)

In this pull request we're adding an optional `tooltip` field. The
`tooltip` field is a string field (with markdown support) that will be
used to display tooltips on hover over app buttons in a workspace
dashboard.

Tooltip screenshot

<img width="816" height="275" alt="Screenshot 2025-08-29 at 4 11 56 PM"
src="https://github.com/user-attachments/assets/52c736a1-f632-465b-89a0-35ca99bd367b"
/>

Tooltip video


https://github.com/user-attachments/assets/21806337-accc-4acf-b8c6-450c031d98f1

Issue: https://github.com/coder/coder/issues/18431
Related provider PR:
https://github.com/coder/terraform-provider-coder/pull/435

### Changes

- Added migration to add `tooltip` column to `workspace_apps` table
- Updated queries to get/set the new `tooltip` column
- Updated frontend to render tooltip as markdown (primary tool tip takes
precedence over template tooltip)

### Testing

- Added storybook test for `Applink` markdown rendering

coderd/database/db2sdk/db2sdk.go

@@ -582,6 +582,7 @@ func Apps(dbApps []database.WorkspaceApp, statuses []database.WorkspaceAppStatus
 			Group:    dbApp.DisplayGroup.String,
 			Hidden:   dbApp.Hidden,
 			OpenIn:   codersdk.WorkspaceAppOpenIn(dbApp.OpenIn),
+			Tooltip:  dbApp.Tooltip,
 			Statuses: WorkspaceAppStatuses(statuses),
 		})
 	}