Add default user-agent 'actions/http-client' when none provided

Co-authored-by: TingluoHuang <1750815+TingluoHuang@users.noreply.github.com>

packages/http-client/__tests__/basics.test.ts

@@ -60,7 +60,7 @@ describe('basics', () => {
     const body: string = await res.readBody()
     const obj = JSON.parse(body)
     expect(obj.url).toBe('https://postman-echo.com/get')
-    expect(obj.headers['user-agent']).toBeFalsy()
+    expect(obj.headers['user-agent']).toBe('actions/http-client')
   })
 
   /* TODO write a mock rather then relying on a third party

packages/http-client/src/index.ts

@@ -816,20 +816,16 @@ export class HttpClient {
     return proxyAgent
   }
 
-  private _getUserAgentWithOrchestrationId(
-    userAgent?: string
-  ): string | undefined {
-    if (!userAgent) {
-      return undefined
-    }
+  private _getUserAgentWithOrchestrationId(userAgent?: string): string {
+    const baseUserAgent = userAgent || 'actions/http-client'
     const orchId = process.env['ACTIONS_ORCHESTRATION_ID']
     if (orchId) {
       // Sanitize the orchestration ID to ensure it contains only valid characters
       // Valid characters: 0-9, a-z, _, -, .
       const sanitizedId = orchId.replace(/[^a-z0-9_.\-]/gi, '_')
-      return `${userAgent} actions_orchestration_id/${sanitizedId}`
+      return `${baseUserAgent} actions_orchestration_id/${sanitizedId}`
     }
-    return userAgent
+    return baseUserAgent
   }
 
   private async _performExponentialBackoff(retryNumber: number): Promise<void> {