docs: optimize link checker

.github/workflows/link_checker_workflow.yaml

@@ -15,6 +15,9 @@ name: Link Checker
 
 on:
   pull_request:
+permissions:
+  contents: read
+  pull-requests: write
 
 
 jobs:
@@ -23,8 +26,33 @@ jobs:
     steps:
       - name: Checkout Repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        with:
+          fetch-depth: 0
+      - name: Identify Changed Files
+        id: changed-files
+        shell: bash
+        run: |
+          git fetch origin main
+          CHANGED_FILES=$(git diff --name-only --diff-filter=ACMRT origin/main...HEAD)
+
+          if [ -z "$CHANGED_FILES" ]; then
+            echo "No markdown files changed. Skipping checks."
+            echo "HAS_CHANGES=false" >> $GITHUB_ENV
+          else
+            echo "--- Changed Files to Scan ---"
+            echo "$CHANGED_FILES"
+            echo "-----------------------------"
+            
+            # Flatten newlines to spaces for the args list
+            FILES_FLAT=$(echo "$CHANGED_FILES" | tr '\n' ' ')
+            
+            echo "CHECK_FILES=$FILES_FLAT" >> $GITHUB_ENV
+            echo "HAS_CHANGES=true" >> $GITHUB_ENV
+          fi
+
 
       - name: Restore lychee cache
+        if: env.HAS_CHANGES == 'true'
         uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
         with:
           path: .lycheecache
@@ -33,6 +61,7 @@ jobs:
 
       - name: Link Checker
         id: lychee-check       
+        if: env.HAS_CHANGES == 'true'
         uses: lycheeverse/lychee-action@a8c4c7cb88f0c7386610c35eb25108e448569cb0 # v2
         continue-on-error: true 
         with:
@@ -42,8 +71,7 @@ jobs:
             --cache
             --max-cache-age 1d
             --exclude '^neo4j\+.*' --exclude '^bolt://.*'
-            README.md
-            docs/
+            ${{ env.CHECK_FILES }}
           output: lychee-report.md
           format: markdown
           fail: true           
@@ -51,18 +79,44 @@ jobs:
           debug: false
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Find comment
+        if: env.HAS_CHANGES == 'true' && steps.lychee-check.outcome == 'failure'
+        uses: peter-evans/find-comment@v3
+        id: find-comment
+        with:
+          issue-number: ${{ github.event.pull_request.number }}
+          comment-author: 'github-actions[bot]'
+          body-includes: "## Link Resolution Note"
+
+      - name: Prepare Report 
+        if: env.HAS_CHANGES == 'true' && steps.lychee-check.outcome == 'failure'
+        run: |
+          # Create a new file 'full-report.md'
+          echo "## Link Resolution Note" > full-report.md
+          echo "Local links and directory changes work differently on GitHub than on the docsite." >> full-report.md
+          echo "You must ensure fixes pass the **GitHub check** and also work with **\`hugo server\`**." >> full-report.md
+          echo "See [Link Checking and Fixing with Lychee](https://github.com/googleapis/genai-toolbox/blob/main/DEVELOPER.md#link-checking-and-fixing-with-lychee) for more details." >> full-report.md
+          echo "---" >> full-report.md
+          echo "### Broken Links Found" >> full-report.md
+          
+          # Clean the report (remove redirects) AND append it to the file
+          sed -E '/(Redirect|Redirects per input)/d' lychee-report.md >> full-report.md
+
+      - name: Create PR Comment
+        if: env.HAS_CHANGES == 'true' && steps.lychee-check.outcome == 'failure'
+        uses: peter-evans/create-or-update-comment@v4
+        with:
+          comment-id: ${{ steps.find-comment.outputs.comment-id }}
+          issue-number: ${{ github.event.pull_request.number }}
+          body-path: full-report.md
+          edit-mode: replace
 
       - name: Display Failure Report
         # Run this ONLY if the link checker failed
         if: steps.lychee-check.outcome == 'failure'
         run: |
-          echo "## Link Resolution Note" >> $GITHUB_STEP_SUMMARY
-          echo "Local links and directory changes work differently on GitHub than on the docsite." >> $GITHUB_STEP_SUMMARY
-          echo "You must ensure fixes pass the **GitHub check** and also work with **\`hugo server\`**." >> $GITHUB_STEP_SUMMARY
-          echo "See [Link Checking and Fixing with Lychee](https://github.com/googleapis/genai-toolbox/blob/main/DEVELOPER.md#link-checking-and-fixing-with-lychee) for more details." >> $GITHUB_STEP_SUMMARY
-          echo "---" >> $GITHUB_STEP_SUMMARY
-
-          echo "### Broken Links Found" >> $GITHUB_STEP_SUMMARY
-          cat ./lychee-report.md >> $GITHUB_STEP_SUMMARY
-
+          # We can now simply output the prepared file to the job summary
+          cat full-report.md >> $GITHUB_STEP_SUMMARY
+          
+          # Fail the job
           exit 1