fix(storage): replace retired azure-storage-blob with maintained azure-blob

[tacoda]

What github issue is this PR for, if any?

Resolves #7094

What changed, and why?

The azure-storage-blob / azure-storage-common gems were retired by Microsoft on 13 Sep 2024 and are unmaintained. They also forced a manual cgi ~> 0.5.1 pin under Ruby 4.0 (azure-storage-common needs CGI.parse, which Ruby 4.0's stripped stdlib omits and Rails 8 no longer pulls in transitively).

Swap to testdouble/azure-blob, a maintained drop-in Active Storage adapter:

• Gemfile: azure-storage-blob -> azure-blob; drop the now-redundant cgi pin (azure-blob declares cgi as a dependency, so the full library resolves again).
• config/storage.yml: microsoft service AzureStorage -> AzureBlob.
• Rewrite the signing regression guard to exercise the AzureBlob Active Storage service (offline URL signing for download + direct upload), replacing the old azure-storage-common internal-class assertions.

Same Azure account / container / keys, so no data migration or infra change.

How is this tested? (please write rspec and jest tests!) 💖💪

Note: if you see a flake in your test build in github actions, please post in slack #casa "Flaky test: " :) 💪
Note: We love capybara tests! If you are writing both haml/js and ruby, please try to test your work with tests at every level including system tests like https://github.com/rubyforgood/casa/tree/main/spec/system

One file touched: spec/lib/azure_storage_signing_spec.rb (rewritten). Dropped tests coupled to methods that are gone with azure-storage-blob. Same intent, re-pointed at the real Active Storage adapter. Builds ActiveStorage::Service::AzureBlobService with dummy credentials.

• It has the full CGI library, not Ruby 4.0's escape-only stdlib.
• It signs a private download URL locally, without hitting Azure.
• It signs a direct-upload URL locally, without hitting Azure.
• Offline: signing is pure-local (SharedKey), no network/live Azure. Runs in CI.
• Higher-level than before: tests the Active Storage service contract, not gem internals → survives future gem/Ruby bumps.
• Result: 3 examples, 0 failures.

Gap (unchanged, flagged earlier)

Still no real round-trip test (upload→store→download against live Azure). This path needs credentials. Covered only by staging smoke-test before merge. App test env uses Disk storage, so the Azure path is exercised nowhere else. That blind spot is exactly why #7093 stayed green.

Needs to go through integration testing through a real Heroku environment to verify a connection to the real Azure account in an environment that is production-like.

Screenshots please :)

Run your local server and take a screenshot of your work! Try to include the URL of the page as well as the contents of the page.

N/A - Backend only change

Feelings gif (optional)

What gif best describes your feeling working on this issue?