[Eng] Move extension test projects under test/extensions

[jviau]

Issue describing the changes in this PR

resolves #issue_for_this_pr

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

Additional information

This PR addresses some mismatching test project organization. We had some extension test projects under /test/ and some under /test/extensions/. This organizes them all under /test/extensions.

There are also some minor changes to msbuild properties:

1. Added some root properties, ExtensionsRoot, TestRoot, SourceRoot
2. Moved SignAssembly and AssemblyOriginatorKeyFile to Directory.Build.props, so all projects will pick these properties up. Previously it was explicitly defined in some test projects.

[Copilot]

Pull Request Overview

This PR reorganizes the test project structure and standardizes build configurations across the codebase. The main changes consolidate test projects under a common test/extensions/ directory, introduce MSBuild property variables for cleaner project references, and centralize assembly signing configuration.

Key changes:

• Moved test projects from test/ to test/extensions/ subdirectory for better organization
• Introduced MSBuild property variables ($(ExtensionsRoot), $(SourceRoot), $(TestRoot)) to simplify project references
• Centralized assembly signing configuration in Directory.Build.props instead of individual project files
• Removed redundant MSBuild properties (IsPackable, IsTestProject, SignAssembly, etc.) from test projects
• Standardized package reference formatting by removing verbose IncludeAssets/PrivateAssets specifications
• Updated CI/CD pipeline configurations to reflect the new test directory structure
• Added LangVersion property to test projects and removed it from some non-test projects

Reviewed Changes

Copilot reviewed 54 out of 96 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Directory.Build.props	Added MSBuild path variables and centralized assembly signing configuration
build/Common.props	Removed assembly signing properties (now centralized)
DotNetWorker.Extensions.sln	Updated solution structure and project references to reflect test reorganization
test/extensions/Worker.Extensions..Tests/.csproj	Standardized test project configurations with cleaner package references and property usage
test/extensions/Worker.Extensions.Tests/*	New consolidated test project with new test files for various extensions
extensions//ci/.yml	Updated CI pipeline paths to point to new test directory structure
Various test project files	Removed or relocated according to new structure

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