test: Add regression coverage for RedisVL CLI foundation contracts (#596)

### Changes
In this PR, we reinforce our test coverage of redisVL CLI foundation
contracts.
For CLI foundation contracts, the following were verified for
correctness:

- sydout/stderr output
- system exit codes
- usage errors
- runtime failures
- human-readable success paths
- JSON success paths

Note: This PR merges commits from:
- fix/cli-exit-codes-and-streams (#592)
- feat/RAAE-1557/cli-json (#593)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Changes are limited to unit tests; they document and guard existing
CLI behavior without modifying runtime code in this diff.
> 
> **Overview**
> This PR **expands CLI regression tests only** (no production
`redisvl/cli` changes in the diff). It replaces shallow fake-Redis tests
with **contract-focused coverage** for `rvl`, `rvl index`, and `rvl
stats`.
> 
> **Top-level CLI** (`test_cli_main.py`): new tests assert help lists
`index`, `mcp`, `version`, and `stats` (in-process and via `python -m
redisvl.cli.runner`), and that unknown commands exit **2** with errors
on stderr and empty stdout.
> 
> **Index CLI** (`test_cli_index.py`): adds help/subprocess help checks,
a real Redis index fixture for `listall`/`info`, success assertions for
`create` (exact success banner; `--json` does not add JSON), JSON shape
checks for `listall` and `info`, human-output smoke tests, and
parametrized **usage errors (exit 2)** vs **runtime/Redis failures (exit
1, no stdout JSON)**.
> 
> **Stats CLI** (`test_cli_stats.py`): mirrors the same patterns—help,
runner subprocess, real-index JSON/table success, `_stats_rows`
ordering, usage **exit 2**, and runtime **`RedisSearchError`/generic
errors exit 1** with empty stdout on `--json` failure paths.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
c7d67fad3917807bd96c71eab53b5d6f56e1dc41. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: ht <121932573+hillarytoh@users.noreply.github.com>
Co-authored-by: Vishal Bala <vishal-bala@users.noreply.github.com>

Author: 3 people