Filter IntelliSense completions by declaration type

We should not be suggesting the concat() function if the contextual/declared type at the current position is not `string` or one of its subtypes.

Start by tracing the IntelliSense completion path for concat() and how it reads the contextual or declared type. Confirm the current suggestions in string and non-string positions, then make the behavior match the stated rule and add or update coverage for those contexts if the relevant tests are found.

Written by the indexing model from the issue text.