This PR migrates service account secret resolution in load-secrets-action from the CLI to the 1Password SDK. The action now selects the backend based on configuration:

• Connect: continue using the 1Password CLI (and only install the CLI in this case).
• Service account (OP_SERVICE_ACCOUNT_TOKEN): resolve op://... references via the 1Password SDK. -> This is new

Changes

• Adds @1password/sdk as a dependency.
• Validates secret references up front using Secrets.validateSecretReference, failing with clearer errors when refs are invalid.
• Expands unit tests to cover the new service-account/SDK path (single + multiple refs, export-env behavior, validation, and error handling).
• Extends e2e workflow coverage with a test ensuring an invalid secret reference fails as expected.

⚠️ The changes in this PR have been previously reviewed, as this is the feature branch.
Reviewed in these PRs: #135, #134

Tests:

To test these changes I created a branch with updates to the update the dist/ folders by running npm run build:all

Branch is here: https://github.com/1Password/load-secrets-action/tree/jill/rebuild-dist

I then created an action in another repo which uses this branch to load secrets and opened a PR. You can see here that both jobs for Service account and Connect ran and were able to load the secrets.

The CLI was not installed for Service Account but was Installed for Connect.

I will wait until we handle to release PR to build the dist files from main once this is merged.