Sort and deduplicate exported packages

[Andrew Barnes (Bortlesboat)]

📖 Description

Sort each exported source's package list by package identifier. For the default
export mode, remove repeated identifiers after sorting. Keep repeated entries
when --include-versions is present so distinct version records are retained.

The workflow tests now start from an intentionally unsorted source and include
a duplicate package, then assert ordering and the mode-specific duplicate
behavior.

AI assistance: OpenAI Codex helped draft the implementation and tests. I
reviewed the complete diff and validation evidence and take responsibility for
the contribution.

🔗 References

Resolves #1058

🔍 Validation

• AppInstallerCLITests.exe 'ExportFlow*' --reporter compact: 38 assertions in 3 test cases passed
• relevant AppInstaller CLI test target built successfully
• git diff --check

✅ Checklist

• Signed the Contributor License Agreement — pending the automated PR check
• Linked to an issue
• Updated Release Notes — not applicable to this focused change
• Updated documentation — not applicable; export behavior and tests are self-contained
• Updated Copilot instructions — not applicable; no build, architecture, or convention change

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

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