FIX: Avoid PR format check workflow label permission failures on fork PRs (#151)

Work Item / Issue Reference
> ADO Workitem:
[AB#38032](https://sqlclientdrivers.visualstudio.com/c6d89619-62de-46a0-8b46-70b92a84d85e/_workitems/edit/38032)

### Summary

The PR Formatting Check workflow fails with a 403 'Resource not
accessible by integration' error when attempting to add size labels to
pull requests from forked repositories. This occurs because the default
GitHub Actions token does not have permission to modify issue labels
when the PR originates from a fork.

## Solution

Added a conditional check to the "Add size label based on PR diff" step
to only run when the PR originates from the main repository, not a fork:

```yaml
- name: Add size label based on PR diff
  if: github.event.pull_request.head.repo.full_name == github.repository
  uses: actions/github-script@v7
  # ... rest of the step
```

## Impact

- ✅ PRs from the main repository continue to work as before (size labels
are added automatically)
- ✅ PRs from forks skip the label-adding step, preventing the 403
permission error
- ✅ All other workflow functionality (title validation, description
validation) remains unchanged for both main repo and fork PRs
- ✅ Contributors from forks can now submit PRs without encountering
workflow failures

## Testing

- Validated YAML syntax is correct
- Confirmed the change is minimal and surgical (single line addition)
- The conditional expression follows GitHub Actions best practices for
fork detection

This fix ensures the workflow remains functional for all contributors
while respecting GitHub's permission model for forked repositories.

*This pull request was created as a result of the following prompt from
Copilot chat.*
> The PR Formatting Check workflow fails with a 403 'Resource not
accessible by integration' error when attempting to add labels to pull
requests from forks. This is due to the default GitHub Actions token not
having permission to modify issue labels from forked repositories.
> 
> To resolve this, update the '.github/workflows/pr-format-check.yml' so
that the step adding labels based on PR diff only runs if the PR
originates from the main repository, not a fork.
> 
> Add the following condition to the label-adding step:
> 
> ```yaml
> - name: Add size label based on PR diff
> if: github.event.pull_request.head.repo.full_name == github.repository
>   uses: actions/github-script@v7
>   with:
>     script: |
>       # ...existing script...
> ```
> 
> This change prevents the workflow from attempting to modify labels
when permissions are insufficient, avoiding the 403 error.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bewithgaurav <8655500+bewithgaurav@users.noreply.github.com>

Author: Copilot

.github/workflows/pr-format-check.yml

@@ -72,6 +72,7 @@ jobs:
               core.setFailed(`❌ PR must contain a meaningful summary section with actual text content (minimum 10 characters).\nPlease add a clear description under the '### Summary' heading in your PR description.`);
             }
       - name: Add size label based on PR diff
+        if: github.event.pull_request.head.repo.full_name == github.repository
         uses: actions/github-script@v7
         with:
           script: |