Fix up snippets and add note

[adegeo]

Summary

• Fixed snippets to render the full sample without snippet comments.
• Added namespaces.
• Added a note that specified the NuGet package required (and pointed out the worker SDK)
• Added a note that says you have to implement your own lifetime manager if using a desktop app framework.

Fixes #46997

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Internal previews

📄 File	🔗 Preview link
docs/core/extensions/dependency-injection/overview.md	.NET dependency injection

[Copilot]

Pull request overview

This PR addresses Issue #46997 by improving the dependency injection documentation examples to be more complete and executable. The changes add full code context with namespaces, using statements, and helpful notes about required packages and desktop application lifetime management.

Changes:

• Added explicit using statements and wrapping <All> snippet tag to Program.cs to show the complete executable example
• Created StandaloneWorker.cs to demonstrate the "bad" pattern of hard-coded dependencies
• Added IMPORTANT notes about the Worker SDK requirement and NuGet package dependencies
• Added guidance for desktop app developers about lifetime management integration

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
docs/core/extensions/dependency-injection/snippets/overview/Program.cs	Added using statements and <All> snippet wrapper to show complete executable code
docs/core/extensions/dependency-injection/snippets/overview/StandaloneWorker.cs	New file demonstrating hard-coded dependency anti-pattern
docs/core/extensions/dependency-injection/overview.md	Added SDK and package notes, updated snippet references to show complete examples