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]

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

[dennisdoomen]

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

See my remark about logging.