Add README status badges for scheduled CI workflows (#4734)

# Description

This adds `README.md` status badges for the five scheduled workflows.
Until now, the README had no badges for them.

| Badge | Workflow | Schedule |
|---|---|---|
| Nightly | `nightly.yml` | Weekly, Sun 02:00 UTC |
| Stress Tests | `stress-test.yml` | Daily 03:00 UTC |
| Connection Stability | `stability-test.yml` | Daily 02:00 UTC |
| Foreign Interop | `interop-foreign.yml` | Daily 03:17 UTC |
| HA Long-Haul | `sample-ha-longhaul.yml` | Weekly, Sun 04:00 UTC |

- Each badge uses `?branch=master&event=schedule`, so it shows only the
result of scheduled runs on `master`. Manual (`workflow_dispatch`) runs
and PR runs don't change the badge.
- Each badge links to that workflow's run list, filtered by
`branch:master event:schedule`.
- The badges go after the existing CI / CodeQL / Coverage badges.
- CodeQL also runs on a schedule, but its existing badge stays as it was
and still covers all runs on `master`.

```markdown
[![Nightly](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/nightly.yml/badge.svg?branch=master&event=schedule)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/nightly.yml?query=branch%3Amaster+event%3Aschedule)
```

## Related Issues

## Checklist

_Put an `x` in the boxes that apply. You can complete these step by step
after opening the PR._

- [ ] I have signed the
[CLA](https://opcfoundation.org/license/cla/ContributorLicenseAgreementv1.0.pdf)
and read the
[CONTRIBUTING](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/CONTRIBUTING.md)
doc.
- [ ] I have added tests that prove my fix is effective or that my
feature works and increased code coverage.
- [x] I have added all necessary documentation.
- [ ] I have verified that my changes do not introduce (new) build or
analyzer warnings.
- [ ] I ran **all** tests locally using the **UA.slnx** solution against
at least .net **framework** and .net **10**, and all passed.
- [ ] I fixed **all** failing and flaky tests in the CI pipelines and
**all** CodeQL warnings.
- [ ] I have addressed **all** PR feedback received.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: romanett <7413710+romanett@users.noreply.github.com>

README.md

@@ -5,6 +5,11 @@
 [![CI](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/buildandtest.yml/badge.svg?branch=master)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/buildandtest.yml?query=branch%3Amaster)
 [![CodeQL](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/codeql-analysis.yml/badge.svg?branch=master)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/codeql-analysis.yml?query=branch%3Amaster)
 [![Coverage](https://codecov.io/gh/OPCFoundation/UA-.NETStandard/branch/master/graph/badge.svg)](https://codecov.io/gh/OPCFoundation/UA-.NETStandard)
+[![Nightly](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/nightly.yml/badge.svg?branch=master&event=schedule)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/nightly.yml?query=branch%3Amaster+event%3Aschedule)
+[![Stress Tests](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/stress-test.yml/badge.svg?branch=master&event=schedule)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/stress-test.yml?query=branch%3Amaster+event%3Aschedule)
+[![Connection Stability](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/stability-test.yml/badge.svg?branch=master&event=schedule)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/stability-test.yml?query=branch%3Amaster+event%3Aschedule)
+[![Foreign Interop](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/interop-foreign.yml/badge.svg?branch=master&event=schedule)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/interop-foreign.yml?query=branch%3Amaster+event%3Aschedule)
+[![HA Long-Haul](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/sample-ha-longhaul.yml/badge.svg?branch=master&event=schedule)](https://github.com/OPCFoundation/UA-.NETStandard/actions/workflows/sample-ha-longhaul.yml?query=branch%3Amaster+event%3Aschedule)
 
 > 🆕 **This is version 2.0 of the OPC UA .NET Standard Stack (current `master`).**
 >