test(ACP): Add missing unit coverage for restore command flows

[samar-703]

Summary

ACP restore commands currently lack direct unit tests for key success and failure paths in:

• packages/cli/src/acp/commands/restore.ts

This creates risk for regressions in checkpoint restore/list behavior and user-facing error messages.

Problem

RestoreCommand and ListCheckpointsCommand include multiple branches that are easy to break without explicit
tests, including:

• checkpointing disabled handling
• missing checkpoint file handling (ENOENT)
• invalid/corrupt checkpoint data handling
• restore stream result formatting
• fallback/unexpected error handling
• checkpoint listing and formatting behavior

Proposed Solution

Add a dedicated test suite:

• packages/cli/src/acp/commands/restore.test.ts

Cover these scenarios:

RestoreCommand

• Delegates to list behavior when invoked without args
• Returns checkpointing-disabled message
• Returns file-not-found message for missing checkpoint
• Returns invalid/corrupt checkpoint message when schema parse fails
• Formats streamed restore results (message, load_history, fallback object)
• Returns generic unexpected error message for non-ENOENT failures

ListCheckpointsCommand

• Returns checkpointing-disabled message
• Returns No checkpoints found. when no .json checkpoints exist
• Formats checkpoint summary output from checkpoint metadata
• Returns generic unexpected error message on failures

Acceptance Criteria

• New restore ACP test file exists and runs successfully.
• All branches above are explicitly asserted.
• Command output strings remain stable and verified by tests.
• No production/runtime code changes are required for this issue.

Validation

Run:

• npm run build --workspace @google/gemini-cli-core
• npm run test --workspace @google/gemini-cli -- src/acp/commands/restore.test.ts

Expected: restore ACP tests pass.

Related PR

Related to #23400