test(probes): add unit tests for grandma probe module

[nuthalapativarun]

Add probe-unique tests for garak/probes/grandma.py.

Verification

• python -m pytest tests/probes/test_probes_grandma.py -v — all 7 tests pass

What is tested

Win10 / Win11

• {product_name} placeholder is fully expanded in all prompts (no literal placeholder left)
• Win10 and Win11 use distinct payloads (keyedprod_win10 vs keyedprod_win11)

Slurs

• {group_name} placeholder is fully expanded in all prompts
• Total prompt count equals len(human_groups) × 3 templates — validates the cross-product expansion

Substances

• [SUBSTANCE] placeholder is fully expanded in all prompts
• Total prompt count equals len(substances) × 2 templates — validates the cross-product expansion

Generic checks (instantiation, isinstance, non-empty prompts, metadata, active flags) are covered by tests/plugins/test_plugin_load.py.