Memory size abbreviations are flagged to expand to eg. "gigabyte" but sometimes should be "gigabyte of"

**Describe the bug**

> At least 1GB free space

In contexts like this one, "1 gigabyte free space" doesn't seem right, unlike the version with "GB". It should be "1 gigabyte of free space".

Start by reproducing the grammar warning with the example “At least 1GB free space” and inspect the rule that expands memory-size abbreviations. Confirm the corrected wording is accepted in this context while other abbreviation expansions remain unchanged, then add or update a regression test for the example.

Written by the indexing model from the issue text.