Handle missing changelog (sections) during pack

[dennisdoomen]

Summary

• Make pack resilient when no changelog release sections are available.
• Add regression coverage for a missing changelog file.
• Clarify that the build entrypoint project is intentionally excluded from solution builds.

Changes

• Handle missing changelog sections during pack
  • Return empty release notes when the changelog file is missing or contains no release sections, avoiding the pack-time exception.
• Adjust solution build settings
  • Keep the solution file aligned with the current build configuration for this PR branch.
  • The build entrypoint project (build/_build.csproj) is intentionally marked as not built in the solution so it is exercised via the build entrypoint (./build.ps1 / ./build.sh) rather than as a normal solution build target.

Combined effect

• Pack no longer fails just because the changelog is absent or empty, and the PR history is split into focused commits.

Validation

• dotnet test tests/Fallout.Common.Tests/Fallout.Common.Tests.csproj --filter ChangelogTasksTest

Note The failing build was caused by #375, but wasn't caught by GHA as that one skips changes to doc files.

[ChrisonSimtian]

ah yes, thats probably the way. Fault tolerant, if its not there skip.
Mind to call the logger to spit out a warning? I'd expect if someone calls the Changelog Task they intend to write it (which in our case was the actual issue, we didnt fully remove it from our internal build process before I killed the Changelog.md)

[ChrisonSimtian]

lgtm, I'd throw a warning ala

Warning: You called the Changelog Task but we couldnt find a changelog.md

And then we can either create one (I'd rather not) or the user at least sees why we didnt write anything