PMC test improvements; remove regions, use a dedicated fixtures folder, improve apex migration skill

[nkolev92]

Bug

This is a test-only follow-up change; no issue required.

Description

Addresses post-merge review feedback from #7467 (PMC Find-Package / Get-Package test migration):

• Removed #region blocks from FindPackageCommandTests.cs and GetPackageCommandTests.cs (Coding Guideline 17).
• Extracted shared test infrastructure into a Fixtures folder — moved the duplicated CmdletRunspaceFixture and TestPSHost into Fixtures/CmdletRunspaceFixture.cs as shared internal classes (registers both Find-Package and Get-Package), eliminating duplication across the two test files. Cleaned up now-unused usings.
• Fixed a comment typo in GetPackageTestCase.cs (falsy -> false).
• Updated the apex-migration skill to reference the repo-wide docs/coding-guidelines.md instead of restating common rules; the Style rules section now only lists migration-specific conventions that supplement the common guidelines.

Verified: NuGetConsole.Host.PowerShell.Test builds with 0 warnings/0 errors and all 23 Find/Get cmdlet unit tests pass.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.