ci: post code coverage delta comment on PRs

.github/workflows/unit-tests.yml

@@ -47,15 +47,94 @@ jobs:
   coverage:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/setup-node@v4
+    - uses: actions/setup-node@v3
       with:
         node-version: 24
-        registry-url: 'https://registry.npmjs.org'
-        cache: 'npm'
-    - run: npm ci
-    - run: npm run coverage
-    - uses: actions/upload-artifact@v4
+
+    - name: Checkout Main
+      uses: actions/checkout@v4
       with:
-        name: coverage-report
-        path: coverage
+        ref: main
+
+    - name: Install & Test (Main)
+      run: |
+        npm ci
+        npm run coverage
+        mv coverage/coverage-summary.json ${{ runner.temp }}/coverage-main-summary.json
+
+    - name: Checkout PR Branch
+      uses: actions/checkout@v4
+      with:
+        clean: true
+
+    - name: Install & Test (PR)
+      run: |
+        npm ci
+        npm run coverage
+
+    - name: Report Coverage Delta
+      uses: actions/github-script@v6
+      with:
+        script: |
+          const fs = require('fs');
+
+          const loadSummary = (path) => {
+            return JSON.parse(fs.readFileSync(path, 'utf8'));
+          };
+
+          let markdown = `### 🧪 Code Coverage Delta\n\n`;
+          markdown += `| Metric | Main | PR | Delta |\n`;
+          markdown += `| :--- | :---: | :---: | :---: |\n`;
+
+          const metrics = ['lines', 'statements', 'functions', 'branches'];
+          const getPct = (summary, metric) => summary.total[metric].pct;
+
+          const mainSummary = loadSummary('${{ runner.temp }}/coverage-main-summary.json');
+          const prSummary = loadSummary('./coverage/coverage-summary.json');
+
+          metrics.forEach(metric => {
+            const oldPct = getPct(mainSummary, metric);
+            const newPct = getPct(prSummary, metric);
+            const diff = (newPct - oldPct).toFixed(2);
+
+            let icon = '';
+            if (diff > 0) icon = '🟢';
+            else if (diff < 0) icon = '🔴';
+            else icon = '⚪️';
+
+            const diffStr = diff > 0 ? `+${diff}%` : `${diff}%`;
+
+            markdown += `| **${metric}** | ${oldPct}% | ${newPct}% | ${icon} ${diffStr} |\n`;
+          });
+
+          markdown += `\n_Generated by [unit-tests.yml](https://github.com/a2aproject/a2a-js/actions/workflows/unit-tests.yml)_`;
+
+          const { owner, repo } = context.repo;
+          const issue_number = context.issue.number;
+
+          const comments = await github.rest.issues.listComments({
+            owner,
+            repo,
+            issue_number,
+          });
+
+          const existingComment = comments.data.find(c => 
+            c.body.includes('Generated by [unit-tests.yml]') && 
+            c.user.type === 'Bot'
+          );
+
+          if (existingComment) {
+            await github.rest.issues.updateComment({
+              owner,
+              repo,
+              comment_id: existingComment.id,
+              body: markdown
+            });
+          } else {
+            await github.rest.issues.createComment({
+              owner,
+              repo,
+              issue_number,
+              body: markdown
+            });
+          }