[CI] Expired Apple development certificates break signing-dependent PR tests

## Summary

The Apple development certificates installed in the PR-test signing keychains expired on July 31, 2026. Signing-dependent tests now fail with:

`No valid iOS/tvOS/macOS code signing keys found in keychain.`

- Stable signature: `expired-apple-development-certificates-no-valid-code-signing-keys`
- Classification: CI infrastructure failure
- Distinct monitored PRs: 4
- Confirmed job occurrences: 17
- First confirmed by the monitor: 2026-07-31T14:42:07Z
- Latest confirmed occurrence: 2026-07-31T16:30:04Z

## Affected PRs

- #26297, head `7e56322bf7c853b1c4ef12626f02cccf36d26fe5`
- [Build 14833205](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14833205), attempt 1
- Failed jobs: `dotnettests_ios`, `dotnettests_tvos`, `dotnettests_macos`, `monotouch_ios`, `monotouch_tvos`, `msbuild`, and Windows `Dotnet tests`
- #26281, head `2df8ff4002b2b5a6d461eeb47a45b9bf576130df`
- [Build 14834283](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834283), attempt 1
- Failed job: [`msbuild`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834283&view=logs&jobId=43070558-71ad-5208-aa3c-4b38cce0e88e)
- #26380, head `85c0366c6c1b1233983943d0d59f08633b291c3c`
- [Build 14834129](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834129), attempt 1
- Failed jobs: [`dotnettests_tvos`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834129&view=logs&jobId=09d26b41-67aa-54a4-f0c7-0e78dc94babc), [`dotnettests_macos`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834129&view=logs&jobId=306fc6d1-8bac-5ab5-4f68-7aa08eedb9c6), [`monotouch_ios`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834129&view=logs&jobId=a70ed34c-97c9-5ebb-c5be-8e2604f5ffcc), [`monotouch_tvos`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834129&view=logs&jobId=1acdac53-199e-5330-5c52-bf5d9d7be24b), and [`msbuild`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834129&view=logs&jobId=43070558-71ad-5208-aa3c-4b38cce0e88e)
- #26381, head `bf31c1874c3da70065394e2cf9a8feadbf722c34`
- [Build 14834130](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834130), attempt 1
- Failed jobs: [`dotnettests_tvos`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834130&view=logs&jobId=09d26b41-67aa-54a4-f0c7-0e78dc94babc), [`monotouch_ios`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834130&view=logs&jobId=a70ed34c-97c9-5ebb-c5be-8e2604f5ffcc), [`monotouch_tvos`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834130&view=logs&jobId=1acdac53-199e-5330-5c52-bf5d9d7be24b), and [`msbuild`](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14834130&view=logs&jobId=43070558-71ad-5208-aa3c-4b38cce0e88e)

## Diagnostic evidence

- `DetectSigningIdentityTaskTests` fails because no valid iOS signing key remains.
- Detailed MSBuild test reports confirm that the installed Apple development certificates are expired.
- iOS, tvOS, and macOS app-build tests then fail in signing detection; the later test-suite summaries are consequences of that same root cause.
- The first failed task in each listed job is `Run tests`. No publication or missing-artifact failure is classified separately.

## Reproduction pattern and impact

Any PR test job that builds a signing-dependent device or extension configuration after the certificate expiry can fail. The issue currently blocks four monitored PRs and affects simulator, Windows-remote, MSBuild, and app-build test jobs across iOS, tvOS, and macOS.

## Investigation performed

The Azure timelines, TestSummary reports, detailed NUnit reports, and representative binlogs were inspected. Searches across open and closed `dotnet/macios` issues used the exact error, `DetectSigningIdentityTaskTests`, keychain-signing terms, and expired-certificate terms. Issues #7171 and #20671 contain the same user-facing error but describe unrelated project/configuration problems, not expired CI credentials.

## Retry status

- #26297: no retry submitted. The durable budget is 3 lifetime retries and 0 for the current head.
- #26281, #26380, and #26381: no retry submitted; their initial builds are still active.
- Retrying before replacement credentials are validated would deterministically repeat the failure.

## Resolution / workaround

Renew and replace the Apple development certificates and associated provisioning profiles in the secure CI signing bundle/keychains. Validate the replacement with a full signing-dependent simulator run. After validation, rerun failed jobs; if Windows Integration is affected, rerun its entire stage.

Azure artifacts and detailed reports are retention-limited.

Start with the DetectSigningIdentityTaskTests failures and the Azure timelines, TestSummary reports, detailed NUnit reports, and representative binlogs named in the issue. Validate replacement signing credentials with a full signing-dependent simulator run, then rerun the affected jobs and confirm the iOS, tvOS, macOS, MSBuild, and Windows test failures are resolved.

Written by the indexing model from the issue text.