Pending PR filter excludes PRs due to skipped iceberg CI checks

## Problem

The [pending PR filter](https://github.com/apache/datafusion-comet/pulls?q=is%3Apr+is%3Aopen+draft%3Afalse+review%3Arequired+status%3Asuccess+sort%3Aupdated-desc) in the README badge uses `status:success`, which excludes PRs that have all checks passing but also have **skipped** iceberg matrix jobs.

The iceberg workflow (`iceberg_spark_test.yml`) defines matrix jobs that skip when the PR title doesn't contain `[iceberg]`. When skipped before matrix expansion, GitHub reports checks with unexpanded names like:

- `iceberg-spark/${{ matrix.os }}/iceberg-${{ matrix.iceberg-version.full }}/...`

These skipped checks cause GitHub to not report the overall commit status as "success", so PRs like #3358 don't appear in the pending PR list despite all real checks passing.

## Possible fixes

1. **Change the filter** from `status:success` to `-status:failure` so PRs are included unless they have actually failing checks
2. **Fix the workflow** to not report skipped checks — e.g. add a gate job that always succeeds and configure branch protection to use that instead of the individual matrix jobs
3. **Move the `if` condition** to the workflow trigger level so no checks are reported at all for non-iceberg PRs

Option 2 is the most robust but requires changing branch protection settings.