proposal: Rework test boundaries and migrate tests to xUnit/Shouldly

[alsi-lawr]

Migrate the test suite from MSTest/VSTest to xUnit and Shouldly, splitting out any remaining unit tests from integration tests.

Motivation

The current tests mix unit coverage with process/RPC integration concerns. This makes the suite harder to reason about and harder to expand safely. MSTest/VSTest are also legacy patterns, and should be modernised.

Scope

• Replace MSTest/VSTest with xUnit.
• Replace assertions with Shouldly.
• Split tests into clear unit/integration boundaries if any remain, and rename tests to follow the standard Given_When_Then scenario naming pattern.
• Keep existing behavioural coverage intact during migration.
• Mark Neovim process/RPC tests as integration tests.
• Ensure CI runs all appropriate test groups.

Acceptance criteria

• Existing tests pass under xUnit.
• Unit and integration tests are clearly separated.
• Integration tests are identifiable/filterable in CI.
• Assertions use Shouldly.

Blocked by

• proposal: Add enforced CSharpier formatting #27
• proposal: Remove vague utility types and migrate to System.Text.Json #28