feat(16-02): add AutotaskClient.getAttachmentContent()

- Fetches Tickets/{id}/Attachments/{attachmentId}, reads response.items?.[0]
- Confirmed live: per-attachment-ID GET is list-shaped, not {item:...}

lib/services/autotask-client.ts

@@ -435,6 +435,26 @@ export class AutotaskClient {
     return response.items || [];
   }
 
+  // Fetches a single attachment's full content (base64 `data` populated).
+  // Confirmed live: the per-attachment-ID GET returns `{items:[...]}`
+  // (list-shaped), NOT `{item:...}` like getEntityById/uploadAttachment —
+  // do not "fix" this to read response.item, that would silently return
+  // undefined content.
+  async getAttachmentContent(
+    entityName: string,
+    entityId: number,
+    attachmentId: number
+  ): Promise<Attachment | null> {
+    const url = `${this.config.apiUrl}/${entityName}/${entityId}/Attachments/${attachmentId}`;
+
+    const response = await this.makeApiCall<ApiResponse<Attachment>>(url, {
+      method: 'GET',
+      headers: this.getAuthHeaders(),
+    });
+
+    return response.items?.[0] ?? null;
+  }
+
   // Time Entries specific methods
   async getTimeEntriesByResource(resourceId: number): Promise<AutotaskTimeEntry[]> {
     return this.queryEntity<AutotaskTimeEntry>('TimeEntries', {