feat: disable packing for test projects and update project files

[ncipollina]

🚀 Pull Request

📋 Summary

Briefly describe what this PR does and why it's needed.

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✅ Checklist

• My changes build cleanly
• I’ve added/updated relevant tests
• I’ve added/updated documentation or README
• I’ve followed the coding style for this project
• I’ve tested the changes locally (if applicable)

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🧪 Related Issues or PRs

Closes #...

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💬 Notes for Reviewers

Any specific areas to look at, known issues, or follow-up work.

[Copilot]

Pull Request Overview

This PR disables packing for test projects and updates project files with additional packaging and documentation settings.

• Added the IsTestProject property to the tests project.
• Updated the src project with detailed NuGet packaging settings and a new SourceLink package reference.
• Set a global rule to disable packing for test projects in Directory.Build.props.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/DynamoDb.DistributedLock.Tests/DynamoDb.DistributedLock.Tests.csproj	Added IsTestProject flag to clearly mark test projects.
src/DynamoDb.DistributedLock/DynamoDb.DistributedLock.csproj	Updated packaging and documentation settings; added SourceLink.
Directory.Build.props	Configured a condition to disable packing for projects flagged as tests.