Fix shebang recommendation in file-based apps docs

[Copilot]

• Update docs/core/sdk/file-based-apps.md Shell Execution section:
  • Change shebang from #!/usr/bin/env dotnet to #!/usr/bin/env -S dotnet --
  • Add explanation of -S flag and -- separator, leading with reason/purpose
  • Add note that #!/usr/bin/env dotnet can be used if -S isn't supported
• Update docs/csharp/fundamentals/tutorials/file-based-programs.md:
  • Revise -S description to accurately say it passes dotnet and -- as separate arguments, while keeping focus on the practical effect

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Internal previews

📄 File	🔗 Preview link
docs/core/sdk/file-based-apps.md	docs/core/sdk/file-based-apps
docs/csharp/fundamentals/tutorials/file-based-programs.md	docs/csharp/fundamentals/tutorials/file-based-programs

[Copilot]

Pull request overview

This PR updates file-based app documentation and snippets to recommend a shebang that reliably forwards command-line arguments to the app (instead of letting dotnet consume arguments like --help).

Changes:

• Updated shebang examples to #!/usr/bin/env -S dotnet -- in the C# tutorial, the SDK docs, and a referenced snippet.
• Added explanation in the SDK docs for why -S and -- are needed for correct argument forwarding.
• Added a fallback note for systems that don’t support env -S.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/csharp/fundamentals/tutorials/snippets/file-based-programs/AsciiArt.cs	Updates the executable shebang in the tutorial snippet to use env -S and -- for correct arg forwarding.
docs/csharp/fundamentals/tutorials/file-based-programs.md	Updates the tutorial’s shebang guidance and adds a tip about fallbacks.
docs/core/sdk/file-based-apps.md	Updates the shebang recommendation and documents the --/-S rationale plus a fallback note.