Describe the bug

The max_tokens configuration set in custom_providers is not being properly passed to the AIAgent instance. Instead, the gateway always uses model.max_tokens from the global config, or falls back to provider-specific defaults (like 32k for Kimi).

To Reproduce

1. Set max_tokens in a custom_provider entry, e.g.:

   custom_providers:
   - name: ark
     max_tokens: 131072
     # ...

2. Remove or don't set model.max_tokens globally
3. Start a conversation through the gateway
4. Observe that max_tokens uses the provider default instead of 131072

Expected behavior
The max_tokens value from custom_providers should be used, with fallback to model.max_tokens if provider-specific config is not present.

Fix
This is fixed by PR #19991 which:

1. Adds max_tokens to _KNOWN_KEYS in config normalization
2. Passes max_tokens through runtime provider resolution
3. Updates gateway to read max_tokens from runtime config first, then fallback to model config

Relates to #19991