Add doc explaining how to test forked PR

docs/fork-pr-testing.md

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+# Fork PR Testing Guide
+
+This document explains how to test external pull requests using the dispatch action workflow system.
+
+## Overview
+
+The testing system consists of three main workflows:
+
+1. **E2E Tests [triggered by maintainer]** (`test-e2e.yml`) - Runs automatically for internal PRs, requires approval for external PRs
+2. **Ok To Test** (`ok-to-test.yml`) - Processes slash commands to trigger fork testing
+3. **E2E tests [fork]** (`test-e2e-fork.yml`) - Triggered manually via slash commands for external PRs
+
+## How It Works
+
+### 1. Initial PR State
+When a PR is created or updated:
+- The `test-e2e.yml` workflow automatically runs
+- For **external PRs**: The `check-external-pr` job detects it's from a fork and **fails intentionally**
+- For **internal PRs**: The workflow proceeds normally with e2e tests
+- External PRs show ❌ for the check-external-pr job with a message asking for maintainer approval
+
+### 2. Manual Approval Required
+**Important**: External PRs require manual approval before workflows can run.
+
+**Steps for maintainers:**
+1. Go to the PR page
+2. Click **"Approve workflow to run"** button
+3. The workflow will then execute
+
+**Note**: The `test-e2e.yml` workflow will still fail for external PRs even after approval, as it's designed to prevent automatic execution. Need to use the slash command instead.
+
+### 3. Testing External PRs
+Once the initial checks have run (and failed), maintainers can test the PR using slash commands:
+
+#### Step-by-Step Process:
+
+1. **Navigate to the PR**
+   - Go to the pull request you want to test
+
+2. **Add a comment with slash command**
+   ```
+   /ok-to-test sha=<commit-sha>
+   ```
+   Replace `<commit-sha>` with the actual commit SHA from the PR.
+   
+   **Note**: Use the short SHA.
+
+3. **Dispatch Action Triggers**
+   - The `ok-to-test.yml` workflow processes the slash command
+   - It triggers a `repository_dispatch` event with type `ok-to-test-command`
+   - The `test-e2e-fork.yml` workflow starts
+
+4. **Workflow Execution**
+   The fork workflow runs two jobs:
+   
+   **a) `e2e-tests` job** (conditional)
+   - Only runs if:
+     - Event is `repository_dispatch`
+     - SHA parameter is not empty
+     - PR head SHA contains the provided SHA
+   - Calls the reusable `e2e-tests.yml` workflow
+   - Runs the actual E2E tests if conditions are met
+   
+   **b) `update-check-status` job** (conditional)
+   - Runs after `e2e-tests` completes
+   - Updates the existing check for job named "e2e-tests"
+   - Sets the conclusion based on `e2e-tests` result:
+     - ✅ **Success** if tests pass
+     - ❌ **Failure** if tests fail
+     - ⏭️ **Skipped** if e2e-tests was skipped
+
+## Troubleshooting
+
+### Workflow Not Running
+- **Check**: Ensure you've approved the workflow to run
+- **Action**: Click "Approve workflow to run" in the PR checks tab
+
+### SHA Not Found
+- **Check**: Verify the SHA exists in the PR commits
+- **Action**: Use `git log --oneline` to find valid commit SHAs
+
+### Dispatch Not Triggering
+- **Check**: Ensure the slash command format is correct
+- **Action**: Use exact format: `/ok-to-test sha=<sha>`
+
+### Check Runs Not Updating
+- **Note**: The fork workflow updates the existing "E2E Tests [reusable]" check run
+- **Behavior**: The same check run is updated with new results rather than creating a new one
+
+## Security Notes
+
+- Only users with **write** permissions can trigger dispatch commands
+- The fork workflow runs in the main repository context, allowing it to update check runs
+- Manual approval is required for external PR workflows
+- External PRs are automatically detected and prevented from running tests automatically
+
+## Workflow Files
+
+- **`.github/workflows/ok-to-test.yml`** - Ok To Test - Slash command processor
+- **`.github/workflows/test-e2e.yml`** - E2E Tests [triggered by maintainer] - Initial PR checks
+- **`.github/workflows/test-e2e-fork.yml`** - E2E tests [fork] - Dispatch action handler
+- **`.github/workflows/e2e-tests.yml`** - E2E Tests [reusable] - Reusable workflow for actual testing
+
+## Testing Checklist
+
+- [ ] PR created with failing check-external-pr check (for external PRs)
+- [ ] Workflow approved to run (if required)
+- [ ] Slash command posted with valid SHA
+- [ ] Dispatch action triggered successfully
+- [ ] E2E Tests [reusable] check run updated with correct conclusion
+- [ ] PR shows both original and updated check runs