[Docs] [C#] Update docs with List<T> returns and IEnumerable<T> tests

[rekhoff]

Description of Changes

• Updates 00500-views.md with:
  • Changes returned value to a nullable value, to match the return type defintion.
  • Wraps sample code in public partial class Module so pasting it in to IDE will not return errors.
• Updates 00500-cheat-sheet.md with:
  • Changes reducer names to not start with On to avoid error STDB0010: Reducer method OnConnect starts with 'On', which is a reserved prefix.
  • Adds Accessor values to views.
  • Updates Filter() returned value to use .ToList() and the return type to use List<Player> rather than IEnumerable<Player> to avoid error, due to needing a return type to be Vec<T> or Option<T>.
• Updated Coddgen.Test to include tests verifying current behavior of IEnumerable<T>, to aid in future update to allow a return type of IEnumerable<T> to be allowed and converted internally to the required type.

API and ABI breaking changes

None

Expected complexity level and risk

1 – documentation-only edits with no behavioral impact.

Testing

• Changed code blocks tested locally to ensure formatting resolved in IDE.

[joshua-spacetime]

Is this change still applicable? #4393 removes Query entirely, right?

[joshua-spacetime]

We should remove any references to Query in the docs and replace with IQuery. llms.md is one such place - see #4394. But I'm not sure if there are others.

[JasonAtClockwork]

Looks good to me!