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/models.go

@@ -4124,6 +4124,8 @@ type WorkspaceApp struct {
 	Hidden       bool               `db:"hidden" json:"hidden"`
 	OpenIn       WorkspaceAppOpenIn `db:"open_in" json:"open_in"`
 	DisplayGroup sql.NullString     `db:"display_group" json:"display_group"`
+	// Markdown text that is displayed when hovering over workspace apps.
+	Tooltip string `db:"tooltip" json:"tooltip"`
 }
 
 // Audit sessions for workspace apps, the data in this table is ephemeral and is used to deduplicate audit log entries for workspace apps. While a session is active, the same data will not be logged again. This table does not store historical data.