fix(azure): drop unused SignalR extension to clear MessagePack CVE-2026-48109

[mrostan]

What & why

Today's vuln scan flagged a new fixable HIGH on latest-azure only:
CVE-2026-48109 — MessagePack 2.5.192 (out-of-bounds read / DoS in the LZ4 path).

The vulnerable MessagePack.dll isn't one of our deps — it's baked into Microsoft's
Azure Functions extension bundle. Its sole consumer is the SignalR Service extension
(verified against the bundle's function.deps.json reverse-dependency graph). Our Azure
Function app only uses HTTP + Durable Functions triggers — never SignalR — so the whole
SignalR assembly cluster is dead weight and MessagePack is reachable code we don't need.

Why delete, not patch the DLL

Docker Scout reports a .NET package only when both its deps.json entry and its DLL
exist on disk. I verified locally:

• Overwriting MessagePack.dll with a patched 2.5.301 build → CVE still reported
  (Scout reads the version from the manifest, not the binary).
• Deleting the DLL → CVE cleared.

So we delete the SignalR-cluster assemblies and de-register the extension. As a bonus this
removes ~6 unused libraries from the image's attack surface, retiring a recurring class of
bundle-CVE findings.

Changes

• resources/azure/prune_signalr_extension.py — deletes the SignalR-cluster DLLs
  (MessagePack(.Annotations), Microsoft.Azure.SignalR.*, …SignalRService) from every
  bundle bin dir and de-registers SignalR from each extensions.json. Fails the build
  loudly if its targets aren't found (a future bundle restructure surfaces as a build error,
  not a silent no-op that lets the CVE back in). Bundle version dir is globbed → survives the
  pending python3.13 base bump.
• Dockerfile.azure — the azure target extracted into its own file (it builds FROM
  Microsoft's Functions base and shares nothing with the python:3.12-slim base stage), with
  the prune wired in. Dockerfile keeps a pointer note; CI now uses -f Dockerfile.azure.
• apollo/interfaces/azure/README.md — fixed the stale Dockerfile_az reference; documented
  the smoke test.
• resources/azure/durable_smoke_test.sh — boots the built image against Azurite and runs a
  Durable Functions orchestration end-to-end.

Validation (local)

• Scout on the built image: 0 MessagePack vulnerabilities; 0 SignalR/MessagePack DLLs remain.
• Smoke test passes: host boots with no SignalR/MessagePack load errors, DurableTask loads on
  the AzureStorage backend, orchestration reaches Completed.

Notes

• Image size is unchanged — deletion clears the scan (scanner reads the merged FS) but the
  base-layer bytes remain; reclaiming them would require flattening, which isn't worth the
  fragility here. The win is vuln/attack-surface reduction, not size.
• Smoke test is intentionally not in CI. The prune's build-time fail-loud runs in both the
  merge-queue and dev-merge builds, and dev deployment exercises Durable Functions for real — so
  both regression modes are already covered. Run the smoke test locally before merging to dev and
  after any base-image change.

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