chore: add testing for A2AExpressApp (#107)

# Description

Add test coverage for A2AExpressApp class.  

Added 19 test cases covering all A2AExpressApp functionality: 
1. Constructor test: Verifies A2AExpressApp instantiates correctly with
requestHandler and
jsonRpcTransportHandler
2. Setup routes default: Tests setupRoutes works with default parameters
and returns the Express app
3. Setup routes custom baseUrl: Tests setupRoutes accepts and uses a
custom baseUrl parameter
4. Setup routes custom middlewares: Tests setupRoutes accepts and
applies custom middleware array
5. Setup routes custom agentCardPath: Tests setupRoutes accepts custom
agentCardPath parameter
6. Agent card GET default: Tests agent card endpoint returns correct
JSON response at default well-known
 path
7. Agent card GET custom path: Tests agent card endpoint works at custom
path when agentCardPath is
specified
8. Agent card error handling: Tests agent card endpoint returns 500
error when getAgentCard fails
9. JSON-RPC single response: Tests POST endpoint handles single JSON-RPC
response correctly
10. JSON-RPC streaming response: Tests POST endpoint handles streaming
responses with Server-Sent Events
11. JSON-RPC streaming error: Tests streaming error handling sends error
event in SSE format
12. JSON-RPC processing error: Tests A2AError handling returns proper
JSON-RPC error response
13. JSON-RPC generic error: Tests non-A2AError fallback returns generic
error response
14. JSON-RPC no ID error: Tests error response includes null ID when
request has no ID
15. Middleware integration: Tests custom middlewares are applied and
called during requests
16. Middleware error handling: Tests middleware errors are properly
handled and return 500
17. Route baseUrl mounting: Tests routes are correctly mounted at
specified baseUrl
18. Route empty baseUrl: Tests routes work correctly when baseUrl is
empty string
19. Express JSON middleware: Tests express.json() middleware is included
by default and parses request
bodies


Dependencies Added:
• Installed supertest and @types/supertest for HTTP endpoint testing

Fixes #105 🦕

Author: ronantakizawa

package-lock.json

@@ -47,6 +47,7 @@
     "@types/mocha": "^10.0.10",
     "@types/node": "^22.13.14",
     "@types/sinon": "^17.0.4",
+    "@types/supertest": "^6.0.3",
     "c8": "^10.1.3",
     "chai": "^5.2.0",
     "express": "^5.1.0",
@@ -55,6 +56,7 @@
     "json-schema-to-typescript": "^15.0.4",
     "mocha": "^11.6.0",
     "sinon": "^20.0.0",
+    "supertest": "^7.1.4",
     "tsup": "^8.5.0",
     "tsx": "^4.19.3",
     "typescript": "^5.8.2"
@@ -83,4 +85,4 @@
   "mocha": {
     "require": "tsx"
   }
-}
+}