ci: Merge PR validation workflows and add reason-specific labels (#5898)

Combine `close-unvetted-pr.yml` and `enforce-draft-pr.yml` into a single
`validate-pr.yml` workflow so the draft enforcer doesn't run on PRs that
were already closed by the vetting check.

**How it works:**
- `validate-non-maintainer-pr` job runs first on `opened` and `reopened`
events, outputs `was-closed` if it closes the PR
- `enforce-draft` job depends on the first job via `needs:` and skips
when `was-closed == 'true'` (uses `if: always()` so it still runs when
the first job is skipped or succeeds without closing)

Each closure reason now also gets a specific label alongside
`violating-contribution-guidelines`:
- `missing-issue-reference` — no issue ref in body, or referenced issues
not found
- `missing-maintainer-discussion` — no prior discussion between author
and a maintainer
- `issue-already-assigned` — referenced issue assigned to someone other
than the PR author

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stephanie-anderson

.github/workflows/enforce-draft-pr.yml

@@ -1,8 +1,8 @@
-name: Close Unvetted Non-Maintainer PRs
+name: Validate PR
 
 on:
   pull_request_target:
-    types: [opened]
+    types: [opened, reopened]
 
 jobs:
   validate-non-maintainer-pr:
@@ -11,6 +11,8 @@ jobs:
     permissions:
       pull-requests: write
       contents: write
+    outputs:
+      was-closed: ${{ steps.validate.outputs.was-closed }}
     steps:
       - name: Generate GitHub App token
         id: app-token
@@ -20,6 +22,7 @@ jobs:
           private-key: ${{ secrets.SDK_MAINTAINER_BOT_PRIVATE_KEY }}
 
       - name: Validate PR
+        id: validate
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea  # v7.0.1
         with:
           github-token: ${{ steps.app-token.outputs.token }}
@@ -101,12 +104,12 @@ jobs:
 
             core.info(`Found ${issueRefs.length} issue reference(s): ${[...seen].join(', ')}`);
 
-            // --- Helper: close PR with comment and label ---
-            async function closePR(message) {
+            // --- Helper: close PR with comment and labels ---
+            async function closePR(message, reasonLabel) {
               await github.rest.issues.addLabels({
                 ...repo,
                 issue_number: pullRequest.number,
-                labels: ['violating-contribution-guidelines'],
+                labels: ['violating-contribution-guidelines', reasonLabel],
               });
 
               await github.rest.issues.createComment({
@@ -120,6 +123,8 @@ jobs:
                 pull_number: pullRequest.number,
                 state: 'closed',
               });
+
+              core.setOutput('was-closed', 'true');
             }
 
             // --- Step 3: No issue references ---
@@ -134,7 +139,7 @@ jobs:
                 '3. Once a maintainer has acknowledged your proposed approach, open a new PR referencing the issue',
                 '',
                 `Please review our [contributing guidelines](${contributingUrl}) for more details.`,
-              ].join('\n'));
+              ].join('\n'), 'missing-issue-reference');
               return;
             }
 
@@ -222,7 +227,7 @@ jobs:
                 'If you believe this assignment is outdated, please comment on the issue to discuss before opening a new PR.',
                 '',
                 `Please review our [contributing guidelines](${contributingUrl}) for more details.`,
-              ].join('\n'));
+              ].join('\n'), 'issue-already-assigned');
               return;
             }
 
@@ -234,7 +239,7 @@ jobs:
                 'To avoid wasted effort on both sides, please discuss your proposed approach in the issue first and wait for a maintainer to respond before opening a PR.',
                 '',
                 `Please review our [contributing guidelines](${contributingUrl}) for more details.`,
-              ].join('\n'));
+              ].join('\n'), 'missing-maintainer-discussion');
               return;
             }
 
@@ -249,4 +254,74 @@ jobs:
               '3. Once a maintainer has acknowledged your proposed approach, open a new PR referencing the issue',
               '',
               `Please review our [contributing guidelines](${contributingUrl}) for more details.`,
-            ].join('\n'));
+            ].join('\n'), 'missing-issue-reference');
+
+  enforce-draft:
+    name: Enforce Draft PR
+    needs: [validate-non-maintainer-pr]
+    if: |
+      always()
+      && github.event.pull_request.draft == false
+      && needs.validate-non-maintainer-pr.outputs.was-closed != 'true'
+    runs-on: ubuntu-24.04
+    permissions:
+      pull-requests: write
+      contents: write
+    steps:
+      - name: Generate GitHub App token
+        id: app-token
+        uses: actions/create-github-app-token@f8d387b68d61c58ab83c6c016672934102569859  # v2
+        with:
+          app-id: ${{ vars.SDK_MAINTAINER_BOT_APP_ID }}
+          private-key: ${{ secrets.SDK_MAINTAINER_BOT_PRIVATE_KEY }}
+
+      - name: Convert PR to draft
+        env:
+          GH_TOKEN: ${{github.token}}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+        run: |
+          gh pr ready "$PR_URL" --undo
+
+      - name: Label and comment
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea  # v7.0.1
+        with:
+          github-token: ${{ steps.app-token.outputs.token }}
+          script: |
+            const pullRequest = context.payload.pull_request;
+            const repo = context.repo;
+
+            // Label the PR so maintainers can filter/track violations
+            await github.rest.issues.addLabels({
+              ...repo,
+              issue_number: pullRequest.number,
+              labels: ['converted-to-draft'],
+            });
+
+            // Check for existing bot comment to avoid duplicates on reopen
+            const comments = await github.rest.issues.listComments({
+              ...repo,
+              issue_number: pullRequest.number,
+            });
+            const botComment = comments.data.find(c =>
+              c.user.type === 'Bot' &&
+              c.body.includes('automatically converted to draft')
+            );
+            if (botComment) {
+              core.info('Bot comment already exists, skipping.');
+              return;
+            }
+
+            const contributingUrl = `https://github.com/${repo.owner}/${repo.repo}/blob/master/CONTRIBUTING.md`;
+
+            await github.rest.issues.createComment({
+              ...repo,
+              issue_number: pullRequest.number,
+              body: [
+                `This PR has been automatically converted to draft. All PRs must start as drafts per our [contributing guidelines](${contributingUrl}).`,
+                '',
+                '**Next steps:**',
+                '1. Ensure CI passes',
+                '2. Fill in the PR description completely',
+                '3. Mark as "Ready for review" when you\'re done'
+              ].join('\n')
+            });