feat(acp): skip Kimi auth check for non-kimi providers

[bu6n]

Summary

• The ACP server's _check_auth method previously always required Kimi OAuth authentication, even when the configured default model used a non-Kimi provider.
• This change checks the provider type of the default model before enforcing Kimi auth. If the provider is not "kimi", the auth check is skipped entirely.

Motivation

Enables using kimi-cli as an ACP server in editors like Zed with a custom provider (e.g., OpenAI-compatible, Anthropic, etc.) without hitting an AUTH_REQUIRED error from the Kimi OAuth flow.

How it works

1. Load the current config.
2. Resolve the default model's provider.
3. If the provider exists and its type is not "kimi", return early from _check_auth.
4. Otherwise, proceed with the existing Kimi OAuth token check.

Testing

• Configure a non-Kimi provider as the default in ~/.kimi/config.toml.
• Start the ACP server and connect from Zed.
• Verify no AUTH_REQUIRED error is raised and completions work as expected.

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