feat: support STORAGE_EMULATOR_HOST environment variable

[SalvatoreT]

Summary

Adds support for the STORAGE_EMULATOR_HOST environment variable to google-cloud-storage, so the client can be pointed at a local Cloud Storage emulator (e.g. fake-gcs-server, the gcloud storage emulator). This brings the Ruby library in line with the Python google-cloud-storage library (see googleapis/google-cloud-python#9219) and the existing emulator-aware Ruby gems (pubsub, datastore, firestore, bigtable, bigquery).

Fixes #34022

Behavior

The emulator can be selected in three ways (in precedence order):

• Google::Cloud::Storage.new emulator_host: "http://localhost:9000"
• Google::Cloud::Storage.configure { |c| c.emulator_host = "http://localhost:9000" }
• STORAGE_EMULATOR_HOST=http://localhost:9000 environment variable

When an emulator host is set:

• It is used as the API endpoint (storage is REST-based, so this sets the underlying root_url).
• Authentication is not required: the client connects anonymously (no Application Default Credentials lookup), but any explicitly-provided credentials/keyfile are still honored.
• A project ID is not required (it is still resolved from the argument / config / GOOGLE_CLOUD_PROJECT when available).

This mirrors the Python client's use of anonymous credentials and an optional project when the emulator is active. Unlike the gRPC-based gems, storage does not use the :this_channel_is_insecure sentinel, since that is a gRPC concept; the REST equivalent is anonymous (nil) credentials, as already used by Google::Cloud::Storage.anonymous.

Changes

• lib/google-cloud-storage.rb: register the emulator_host config field (defaulting to STORAGE_EMULATOR_HOST) and thread emulator_host: through the Google::Cloud.storage / Google::Cloud#storage convenience methods.
• lib/google/cloud/storage.rb: add the emulator_host: keyword to Storage.new, connect anonymously to the emulator without requiring credentials or a project, and document the option.
• test/google/cloud/storage_test.rb: unit tests covering the env var, the keyword argument, the config field, the optional project, and honoring explicitly-provided credentials.

Testing

• test/google/cloud/storage_test.rb: 22 runs, 303 assertions, 0 failures.
• Rubocop clean on the changed library files.

🤖 Generated with Claude Code

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[SalvatoreT]

Hey @cpriti-os @aandreassa @bajajneha27 if y'all are open to externally-authored PRs, would one of you be able to review this?