[CI Failure] Infrastructure.Tests (ubuntu-latest) job fails with generic exit code 2, unrelated to PR changes

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/34740434278
Build error leg: Tests / No-package tests (regular, Infrastructure.Tests, Infrastructure, Infrastructure, tests/Infrastruc... / Infrastructure (ubuntu-latest)
Pull request: #20020

## Error Message

```
Process completed with exit code 2. (Run tests (Linux/macOS) step failed; job logs unavailable: 'Failed to fetch logs for job'). SelectTests warning: git merge-base exited 1 (no common ancestor) -- fell back to running ALL tests for the PR, which can surface unrelated pre-existing failures. No TRX test failures extracted. PR only modifies extension/ files, unrelated to Infrastructure.Tests.
```

## Description

Infrastructure.Tests (ubuntu-latest) job fails with generic exit code 2, unrelated to PR changes

**Type**: flaky-test

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-09-13 | [34740434278](https://github.com/microsoft/aspire/actions/runs/34740434278) | Tests / No-package tests (regular, Infrastructure.Tests, Infrastructure, Infrastructure, tests/Infrastruc... / Infrastructure (ubuntu-latest) | #20020 |

Start with build 34740434278 and the Tests / No-package tests Infrastructure (ubuntu-latest) job, then inspect the SelectTests warning and PR #20020. Determine why git merge-base reports no common ancestor and why the job exits 2 without logs; done means the unrelated Infrastructure.Tests failure is diagnosed and CI no longer surfaces it for this case.

Written by the indexing model from the issue text.