Fix implicit string concat and skeleton 404 mapping

Address the review on PR #220. Wrap each retrieval-rag CORPUS passage in
parentheses so the multi-line string is an explicit grouped expression,
not adjacent string literals implicitly concatenated inside the list (a
CodeQL missing-comma footgun in a copy-me example). Add the
404 -> ProviderInvalidModel branch and import to the authoring-guide
skeleton's _classify(), which contradicted its own contract checklist.

Author: chris-colinsky

docs/retrieval-providers/authoring.md

@@ -23,6 +23,7 @@ import httpx
 
 from openarmature.llm import (
     ProviderAuthentication,
+    ProviderInvalidModel,
     ProviderInvalidRequest,
     ProviderInvalidResponse,
     ProviderRateLimit,
@@ -99,6 +100,8 @@ def _classify(resp: httpx.Response) -> Exception:
     status = resp.status_code
     if status in (401, 403):
         return ProviderAuthentication(f"HTTP {status}")
+    if status == 404:
+        return ProviderInvalidModel("model not found")
     if status == 429:
         return ProviderRateLimit("HTTP 429")
     if status in (400, 413, 422):

examples/retrieval-rag/main.py

@@ -84,22 +84,38 @@
 # self-contained. The passages deliberately overlap in topic so reranking
 # has something to disambiguate.
 CORPUS: list[str] = [
-    "The Moon's south pole holds water ice in permanently shadowed crater floors "
-    "that never see sunlight, a candidate resource for future crews.",
-    "Apollo 11 landed in the Sea of Tranquility in July 1969; Armstrong and Aldrin "
-    "spent about two and a half hours walking on the surface.",
-    "The lunar maria are vast basaltic plains that formed when ancient impact basins "
-    "flooded with lava, giving the near side its dark patches.",
-    "A lunar day lasts about 29.5 Earth days, so any point on the surface sees roughly "
-    "two weeks of sunlight followed by two weeks of night.",
-    "The Moon is slowly receding from Earth at about 3.8 centimeters per year, measured "
-    "by bouncing lasers off the retroreflectors Apollo crews left behind.",
-    "Regolith, the layer of loose dust and broken rock covering the Moon, is abrasive "
-    "and clings to everything, a persistent hazard for equipment and spacesuits.",
-    "Apollo 13 aborted its landing after an oxygen tank ruptured; the crew looped around "
-    "the Moon and returned safely using the lunar module as a lifeboat.",
-    "Permanently shadowed regions near the poles are among the coldest places in the "
-    "solar system, cold enough to trap water ice for billions of years.",
+    (
+        "The Moon's south pole holds water ice in permanently shadowed crater floors "
+        "that never see sunlight, a candidate resource for future crews."
+    ),
+    (
+        "Apollo 11 landed in the Sea of Tranquility in July 1969; Armstrong and Aldrin "
+        "spent about two and a half hours walking on the surface."
+    ),
+    (
+        "The lunar maria are vast basaltic plains that formed when ancient impact basins "
+        "flooded with lava, giving the near side its dark patches."
+    ),
+    (
+        "A lunar day lasts about 29.5 Earth days, so any point on the surface sees roughly "
+        "two weeks of sunlight followed by two weeks of night."
+    ),
+    (
+        "The Moon is slowly receding from Earth at about 3.8 centimeters per year, measured "
+        "by bouncing lasers off the retroreflectors Apollo crews left behind."
+    ),
+    (
+        "Regolith, the layer of loose dust and broken rock covering the Moon, is abrasive "
+        "and clings to everything, a persistent hazard for equipment and spacesuits."
+    ),
+    (
+        "Apollo 13 aborted its landing after an oxygen tank ruptured; the crew looped around "
+        "the Moon and returned safely using the lunar module as a lifeboat."
+    ),
+    (
+        "Permanently shadowed regions near the poles are among the coldest places in the "
+        "solar system, cold enough to trap water ice for billions of years."
+    ),
 ]
 
 # How many candidates cosine retrieval hands to the reranker, and how many