Fix unit test types for context item to be compatible with LS changes

Description

This language server MR had breaking changes to the AIContextItem types.

In chore: update mock data to support updated LS t... (!2085 - merged) I preemptively fixed those types before the language server change was merged, but unfortunately missed one. This results in a compiler error within a unit test.

This MR updates the remaining mock context item to use createFakePartial, which allows a more permissive object which matches the new LS types.

Note that this issue is currently only reproducible when linking latest LS into the VS codebase via the watch script, not via the currently installed LS package.

Related Issues

Resolves #1661 (closed)

How has this been tested?

Code compiles with main, and also with latest LS main linked into the VS project.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes