Make `cds get` manifest parsing fail-safe and observable

[RonaldHensbergen]

Summary

When .cds/get-manifest.json is invalid JSON or malformed, current behavior silently resets to an empty manifest.

Problem

• Silent reset hides corruption.
• Prior tracking metadata can be lost without visibility.
• Troubleshooting is harder when state is replaced without warning.

Proposed change

• On malformed manifest, make the event observable (warning/error).
• Prefer a fail-safe strategy such as backup-and-reset or explicit failure with guidance.
• Keep normal behavior unchanged for valid manifests.

Acceptance criteria

• Malformed manifest handling is explicit and user-visible.
• Existing valid manifests continue to load/write as before.
• Metadata loss is prevented or clearly communicated during recovery behavior.