ci: use esbuild to ensure client and root entry points build for browser (#210)

# Description

Add an automated test for the issue solved previously in #46 to ensure
that client and root entry points build for browsers.

Example failure would look like:

```
✘ [ERROR] Could not resolve "events"

    dist/server/index.js:27:29:
      27 │ import { EventEmitter } from "events";
         ╵                              ~~~~~~~~

  The package "events" wasn't found on the file system but is built into node. Are you trying to
  bundle for node? You can use "--platform=node" to do that, which will remove this error.
```

(obtained by running `esbuild ./dist/server/index.js --bundle
--platform=browser --outdir=dist/tmp-checks --outbase=./dist`)

---------

Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

Author: ishymko

.github/workflows/build-tests.yml

@@ -0,0 +1,25 @@
+# Ensures no accidential Node.js API usage in browser-compatible entrypoints.
+
+name: Run Build Tests
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  test:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-node@v4
+      with:
+        node-version: 18
+        registry-url: 'https://registry.npmjs.org'
+        cache: 'npm'
+    - run: npm ci
+    - run: npm run build
+    - run: npm run test-build:browser

package-lock.json

@@ -51,6 +51,7 @@
     "@types/supertest": "^6.0.3",
     "c8": "^10.1.3",
     "chai": "^5.2.0",
+    "esbuild": "^0.27.0",
     "eslint": "^9.39.1",
     "eslint-config-prettier": "^10.1.8",
     "eslint-plugin-prettier": "^5.5.4",
@@ -80,7 +81,8 @@
     "a2a:cli": "tsx src/samples/cli.ts",
     "agents:movie-agent": "tsx src/samples/agents/movie-agent/index.ts",
     "agents:sample-agent": "tsx src/samples/agents/sample-agent/index.ts",
-    "agents:extension-agent": "tsx src/samples/extensions/index.ts"
+    "agents:extension-agent": "tsx src/samples/extensions/index.ts",
+    "test-build:browser": "esbuild ./dist/client/index.js ./dist/index.js --bundle --platform=browser --outdir=dist/tmp-checks --outbase=./dist"
   },
   "dependencies": {
     "uuid": "^11.1.0"