chore: Re-enable Candid compatibility checks

[aterga]

Motivation

These checks were temporarily disabled in #3298

Changes

Uncommented interface-compatibility in .github/workflows/canister-tests.yml

Tests

Let's see

[Copilot]

Pull Request Overview

This PR re-enables Candid compatibility checks that were temporarily disabled in a previous pull request. The change restores the automated verification that ensures the Internet Identity canister interface remains compatible with the latest released version.

• Re-enables the interface-compatibility GitHub Actions job
• Restores automated Candid interface compatibility validation using didc

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[aterga]

Closing this PR as stale — it has been open for ~7 months with no review activity. The Candid compatibility checks are still commented out in CI on main. If re-enabling them is still desired, a fresh PR against current main would be more appropriate since the CI workflow has likely evolved.

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