fix(provider): iflow provider should use merged provider config (#751)

Fix a bug where iflow provider's createModel function was using the
static iflowProvider object instead of the merged _provider parameter.
This prevented the provider from reading apiKey from config file
(provider.iflow.options.apiKey), causing requests to be sent without
x-iflow-signature header when IFLOW_API_KEY env var was not set.

Changes:
- Use _provider instead of iflowProvider in getProviderApiKey call
- Use _provider instead of iflowProvider in createModelCreator call

This ensures the provider respects user configuration and properly
generates authentication signatures for all API requests.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: jhao0413

src/provider/providers/iflow.ts

@@ -38,7 +38,7 @@ export const iflowProvider: Provider = {
     'kimi-k2.5': {},
   },
   createModel(name, _provider, options) {
-    const apiKey = getProviderApiKey(iflowProvider);
+    const apiKey = getProviderApiKey(_provider);
     const baseFetch = options.customFetch ?? fetch;
     const customFetch = (async (url: RequestInfo | URL, init?: RequestInit) => {
       const headers = new Headers(init?.headers);
@@ -61,7 +61,7 @@ export const iflowProvider: Provider = {
         headers: Object.fromEntries(headers.entries()),
       });
     }) as typeof fetch;
-    return createModelCreator(name, iflowProvider, {
+    return createModelCreator(name, _provider, {
       ...options,
       customFetch,
     });