Minor cleanups after PR #560 (`throw` style, `Features` casing, stray whitespace)

[raandree]

Problem description
Three low-priority cleanups noticed during review of #560:

1. throw style inconsistency. The new Plaster v1/v2 fallback in Sampler/Public/Add-Sample.ps1 and Sampler/Public/New-SamplerPipeline.ps1 uses a plain string throw. Sampler elsewhere uses $script:localizedData + a structured ErrorRecord, and the new public-functions.instructions.md / private-functions.instructions.md prescribe that style.
2. Features ValidateSet casing. The list mixes casing deliberately (Enum, Classes, git, gitversion, github, vscode, codecov, azurepipelines, Gherkin, UnitTests, ModuleQuality, Build, AppVeyor, TestKitchen). PowerShell binding and -contains in the manifest are case-insensitive, so it works, but the asymmetry is ugly and invites typos in downstream contributions.
3. Stray whitespace. tests/Integration/PlasterTemplates/Sampler/CustomModulePlasterTemplate.Integration.Tests.ps1 lost a space before a trailing comment: Features = 'git'# All,Enum,Classes,....

Verbose logs
N/A

How to reproduce
Read the files listed above.

Expected behavior

• Structured ErrorRecord throws with localized messages.
• Consistent casing across ValidateSet and manifest <choice value="..."> entries.
• Space before # comment.

Current behavior
Plain-string throw; mixed casing; missing space before #.

Suggested solution
Address each item in a single small cleanup PR; none of them are urgent.

Operating system the target node is running
N/A

PowerShell version and build the target node is running
N/A

Module version used
Sampler Unreleased / post-#560.