BUG: Use case-correct /CID/ path in PrefetchCIDContentLinks.py

The GitHub Pages mirror published from the gh-pages branch of
InsightSoftwareConsortium/ITKTestingData stores blobs under
/CID/<cid> (uppercase), matching the convention used by every
other algo bucket (/MD5/, /SHA512/) and the CMake-side
ExternalData URL template that resolves %(algo) to the uppercase
file extension.  PrefetchCIDContentLinks.py used /cid/ (lowercase)
which 404s on case-sensitive GitHub Pages, causing the prefetch
to silently fall through to public IPFS gateways for every CID.

Recently-uploaded testing-data blobs (Montage, GrowCut, AnisotropicDiffusionLBR,
GenericLabelInterpolator, etc.) live ONLY on the GitHub Pages
mirror — they are not pinned to public IPFS networks — so the
typo turned every dweb.link 504 into a hard prefetch failure for
those 71 CIDs and produced the "refusing to save a partial cache"
job error in the populate-externaldata-cache workflow.

Verified by direct curl:
  /cid/<cid>  -> HTTP 404 (9 KB error page)
  /CID/<cid>  -> HTTP 200 (full blob, e.g. 2.5 MB for
                bafkreic6bv4q6zngkeychplskxwabwr3p2nukhtda2bdp26a2zmfkfyil4)

Author: hjmjohnson

Utilities/Maintenance/PrefetchCIDContentLinks.py

@@ -30,7 +30,7 @@
 # comes first because it is the lowest-latency bulk-hosted option and does
 # not rate-limit CI.
 GATEWAYS = (
-    "https://insightsoftwareconsortium.github.io/ITKTestingData/cid/{cid}",
+    "https://insightsoftwareconsortium.github.io/ITKTestingData/CID/{cid}",
     "https://ipfs.io/ipfs/{cid}",
     "https://gateway.pinata.cloud/ipfs/{cid}",
     "https://cloudflare-ipfs.com/ipfs/{cid}",