add rank task

Here, the `rank` task includes both the retrieval stage and the re-rank stage in the searching process.

The issue names no files, tests, or entry points. Start by locating the existing task definitions and the retrieval and re-ranking evaluation flow, then determine how a rank task should be registered. Done means the task covers both stages as described and its behavior is verified by the project’s relevant checks.

Written by the indexing model from the issue text.