fix(aihubmix): switch to dedicated provider package to resolve Zod validation error

Replace @ai-sdk/openai-compatible with the official aihubmix/ai-sdk-provider
package and remove the hardcoded api URL, as the dedicated package handles
schema validation and endpoint configuration internally.

Author: xinrui-z

providers/aihubmix/provider.toml

@@ -1,6 +1,5 @@
 name = "AIHubMix"
-npm = "@ai-sdk/openai-compatible"
+npm = "aihubmix/ai-sdk-provider"
 env = ["AIHUBMIX_API_KEY"]
-api = "https://aihubmix.com/v1"
 doc = "https://docs.aihubmix.com"