Prevent fallback to text-based completions when completing object properties with unknown declared type

**Bicep version**
Unknown

**Describe the bug**
This is related to #14065. We appear to be falling back to text-based completions.

**To Reproduce**
n/a

**Additional context**
n/a

Start by reading the related issue #14065 and reviewing the attached completion example. No source files, tests, or reproduction steps are named, so locate the object-property completion entry point before determining how unknown declared types are handled. Done means text-based completions are no longer used for this case and the behavior is covered by an appropriate test.

Written by the indexing model from the issue text.