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.

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