Assert PROMPT_GROUP_INVALID constant in category test

Address CoPilot review on #228: test_error_categories_match_spec pinned
PromptGroupInvalid.category but not the exported PROMPT_GROUP_INVALID
constant, so a regression in the constant value or export could slip
through. Add the constant assertion (and its import) alongside the
other three categories.

Test-only; no behavior change.

Author: chris-colinsky

tests/unit/test_prompts.py

@@ -20,6 +20,7 @@
 
 from openarmature.llm.messages import Message, UserMessage
 from openarmature.prompts import (
+    PROMPT_GROUP_INVALID,
     PROMPT_NOT_FOUND,
     PROMPT_RENDER_ERROR,
     PROMPT_STORE_UNAVAILABLE,
@@ -56,6 +57,7 @@ def test_error_categories_match_spec() -> None:
     assert PROMPT_NOT_FOUND == "prompt_not_found"
     assert PROMPT_RENDER_ERROR == "prompt_render_error"
     assert PROMPT_STORE_UNAVAILABLE == "prompt_store_unavailable"
+    assert PROMPT_GROUP_INVALID == "prompt_group_invalid"
 
 
 def test_transient_categories_contains_only_store_unavailable() -> None: