Cli dx improvements

[xav-db]

Greptile Summary

This PR makes two DX improvements to the Helix CLI: it allows helix add to run inside a project that currently has zero instances (so users can re-add an instance after deleting the last one), and it adds a new helix cluster indexes subcommand that lists vector, equality, and range indexes for an Enterprise cluster.

• Zero-instance tolerance for helix add: HelixConfig::from_file and ProjectContext::find_and_load are each split into a strict and a lenient variant; helix add now uses the lenient path, with new unit tests covering both paths.
• helix cluster indexes command: A new Indexes subcommand (aliased indices) is added under ClusterConfigAction; it auto-resolves the cluster ID from the current project's Enterprise instance when --cluster-id is omitted, and supports --format json.
• Improved error hint: The MissingInstances error now surfaces a concrete helix add command as a hint.

Important Files Changed

Filename	Overview
helix-cli/src/commands/config.rs	Adds list_indexes_for_cluster handler and resolve_cluster_id_for_indexes fallback; error from find_and_load is silently swallowed, which can hide config-parse failures behind a generic hint message.
helix-cli/src/enterprise_cloud.rs	Adds CliClusterIndex/CliClusterIndexes structs and fetch_indexes_for_cluster; alias-only rename means --format json serializes to index_name/index_type instead of the API's name/type.
helix-cli/src/config.rs	Cleanly refactors from_file into a from_file_inner(require_instances) gate; new tests cover both strict and lenient validation paths.
helix-cli/src/project.rs	Adds find_and_load_allow_no_instances by delegating to a shared load_with(require_instances) helper; logic is straightforward.
helix-cli/src/commands/add.rs	Switches to find_and_load_allow_no_instances so helix add can re-add an instance when the last one was deleted.
helix-cli/src/lib.rs	Adds Indexes subcommand under ClusterConfigAction with --cluster-id/--format args and indices alias; clean.
helix-cli/src/errors.rs	Adds actionable with_hint to MissingInstances error message; no logic changes.
helix-cli/src/main.rs	Adds a parser test for cluster indexes --cluster-id ent_123; straightforward test coverage.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[helix cluster indexes] --> B{--cluster-id provided?}
    B -- yes --> C[trim & validate non-empty]
    C --> D[Use provided cluster_id]
    B -- no --> E[ProjectContext::find_and_load\nrequire_instances=true]
    E -- error --> F[Return: Provide --cluster-id hint]
    E -- ok --> G{enterprise instances count}
    G -- 0 --> H[Error: No Enterprise instances\nin helix.toml]
    G -- 1 --> I[Use sole instance cluster_id]
    G -- many & interactive --> J[prompts::select_instance]
    G -- many & non-interactive --> K[Error: list available instances]
    J --> L[Lookup cluster_id from config]
    I --> L
    D --> M[require_auth + fetch_indexes_for_cluster]
    L --> M
    M --> N{--format json?}
    N -- yes --> O[print_json]
    N -- no --> P[print_cluster_indexes\nvector / equality / range]

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_{Reviews (1): Last reviewed commit: "Implement lenient configuration loading ..." | Re-trigger Greptile}

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