Extract _download_manifest to dedupe the IWC fetch block

fetch_manifest and refresh_manifest both had a byte-identical
fetch/raise_for_status/json/isinstance block. Pull it into a private
helper so the two functions differ only in their locking strategy --
which is the actual point refresh_manifest's docstring labors to explain.

lib/galaxy/agents/iwc.py

@@ -33,6 +33,16 @@ def clear_manifest_cache() -> None:
         _manifest_cache.clear()
 
 
+def _download_manifest(timeout: float) -> list[dict[str, Any]]:
+    """Fetch and validate the IWC manifest over the network, bypassing the cache."""
+    response = requests.get(IWC_MANIFEST_URL, timeout=timeout)
+    response.raise_for_status()
+    manifest = response.json()
+    if not isinstance(manifest, list):
+        raise ValueError(f"IWC manifest at {IWC_MANIFEST_URL} did not return a JSON array")
+    return manifest
+
+
 def fetch_manifest(timeout: float = 30.0) -> list[dict[str, Any]]:
     """Fetch the IWC manifest, returning a cached copy when fresh.
 
@@ -44,11 +54,7 @@ def fetch_manifest(timeout: float = 30.0) -> list[dict[str, Any]]:
         if cached is not None:
             return cached
 
-        response = requests.get(IWC_MANIFEST_URL, timeout=timeout)
-        response.raise_for_status()
-        manifest = response.json()
-        if not isinstance(manifest, list):
-            raise ValueError(f"IWC manifest at {IWC_MANIFEST_URL} did not return a JSON array")
+        manifest = _download_manifest(timeout)
         _manifest_cache[_CACHE_KEY] = manifest
         return manifest
 
@@ -66,11 +72,7 @@ def refresh_manifest(timeout: float = 30.0) -> list[dict[str, Any]]:
     continue without the data); this one is called from a periodic task
     that has to tolerate transient failure.
     """
-    response = requests.get(IWC_MANIFEST_URL, timeout=timeout)
-    response.raise_for_status()
-    manifest = response.json()
-    if not isinstance(manifest, list):
-        raise ValueError(f"IWC manifest at {IWC_MANIFEST_URL} did not return a JSON array")
+    manifest = _download_manifest(timeout)
     with _manifest_cache_lock:
         _manifest_cache[_CACHE_KEY] = manifest
     return manifest