chore(ci): Update CloudFlare previews to run on adding label (#2901)

## Description

This PR adds a label check for the PR Previews on cloudflare.

This is required as CloudFlare only allows 500 deployments in a month.
By adding this label we can regulate the number of deployments per PR.

## PR Checklist

> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

Author: dishaprakash

.github/workflows/docs_preview_build_cf.yaml

@@ -17,6 +17,7 @@ name: "CF: Build Docs Preview"
 
 on:
   pull_request:
+    types: [opened, synchronize, reopened, labeled]
     paths:
       - 'docs/**'
       - '.github/workflows/docs*_cf.yaml'
@@ -26,6 +27,7 @@ permissions: read-all
 
 jobs:
   build-preview:
+    if: "contains(github.event.pull_request.labels.*.name, 'docs: deploy-preview')"
     runs-on: ubuntu-24.04
     env:
       PR_NUMBER: ${{ github.event.number }}
@@ -86,4 +88,27 @@ jobs:
           echo "The build for PR #${{ env.PR_NUMBER }} succeeded." >> $GITHUB_STEP_SUMMARY
           echo "The Cloudflare deployment workflow is now starting." >> $GITHUB_STEP_SUMMARY
           echo "---" >> $GITHUB_STEP_SUMMARY
-          echo "#### [Track Deployment Progress]($DEPLOY_URL)" >> $GITHUB_STEP_SUMMARY
+          echo "#### [Track Deployment Progress]($DEPLOY_URL)" >> $GITHUB_STEP_SUMMARY
+
+  remove-label:
+    needs: build-preview
+    if: "always() && contains(github.event.pull_request.labels.*.name, 'docs: deploy-preview')"
+    runs-on: ubuntu-24.04
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Remove deploy-preview label
+        uses: actions/github-script@ed597411d8f924073f98dfc5c65a23a2325f34cd # v8
+        with:
+          script: |
+            try {
+              await github.rest.issues.removeLabel({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: context.issue.number,
+                name: 'docs: deploy-preview'
+              });
+              console.log("Label 'docs: deploy-preview' removed successfully.");
+            } catch (error) {
+              console.log(`Error removing label: ${error}`);
+            }

DEVELOPER.md

@@ -382,6 +382,7 @@ go test -race -v ./cmd/... ./internal/...
   workflows on your PR.
   * Maintainers can comment `/gcbrun` to execute the integration tests.
   * Maintainers can add the label `tests:run` to execute the unit tests.
+  * Maintainers can add the label `docs: deploy-preview` to run the PR Preview workflow.
 
 #### Test Resources