fix(moonshot): update discontinued Kimi model IDs

## Bug Report

The Moonshot provider currently uses `KIMI_K2_5` (`kimi-k2.5`) in its documentation example and provider smoke tests.

That model ID has been discontinued, so credential-gated tests using it can fail with a 404.

Moonshot's current K2-family model is `kimi-k2.6`.

## Expected behavior

Moonshot examples and smoke tests should use a currently supported model ID.

## Proposed fix

- add a `KIMI_K2_6` constant;
- update the Moonshot example to use it;
- update the Moonshot provider smoke tests to use it;
- keep the existing `KIMI_K2` and `KIMI_K2_5` constants exported for compatibility.

Find the Moonshot provider files, likely under a providers/ or moonshot/ directory. Look for constants like KIMI_K2_5 in the code and documentation examples. Update the constant definition and its usage in smoke tests. Run the provider's tests to verify the change works and doesn't break compatibility.

Scritto dal modello di indicizzazione a partire dal testo della issue.