fix(logs): support index option

[Platane]

What does this PR do?

Adds --index support to pup logs search, pup logs list, pup logs query, and pup logs aggregate.

The option uses Clap's Vec<String> + value_delimiter = ',' pattern, so callers can pass either --index a,b or repeated --index a --index b.

Motivation

The CLI accepted --index for logs search, but the value was dropped before reaching the Datadog API.

The --index option is particularly useful to route BYOC Logs search queries.

Additional Notes

Validation:

• cargo fmt
• cargo test commands::logs
• cargo build
• cargo clippy -- -D warnings

Validated against the dogfooding org from the local source build:

• cargo run -- logs query --query '*' --index byoc--test--main --from 15min --limit 20 ✅
• cargo run -- logs aggregate --query '*' --index byoc--test--main --from 15min --compute count --group-by status --limit 20 ✅

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

N/A