Add CI check to detect dependency changes that may affect downstream users

[drganjoo]

Summary

When a crate removes a dependency or removes a feature flag on an existing dependency, downstream users who relied on transitive feature enablement may encounter compilation errors. We should add a CI check to detect such changes and ensure they are documented in release notes.

Background

Rust's feature unification combines all features across the dependency graph. If crate A depends on crate B with feature X enabled, users of A also get feature X on B without explicitly requesting it.

Two types of changes can break downstream users:

1. Removing a dependency that was enabling features on other crates
2. Removing a feature flag on an existing dependency

In both cases, users see cryptic compilation errors that do not indicate a missing feature flag is the root cause.

Proposed Action

1. Add a CI check that parses Cargo.toml diffs and detects:
   • Removed dependencies
   • Removed features on existing dependencies
2. When such changes are detected, verify that a corresponding changelog entry exists
3. Fail the CI check if the changelog does not document the change

Acceptance Criteria

• CI check detects removed dependencies in Cargo.toml
• CI check detects removed features on existing dependencies
• CI check verifies a changelog entry exists for detected changes
• CI check fails if changelog entry is missing, passes otherwise