feat: add query_params option to ModelProviderInfo to support Azure (#1435)

As discovered in https://github.com/openai/codex/issues/1365, the Azure
provider needs to be able to specify `api-version` as a query param, so
this PR introduces a generic `query_params` option to the
`model_providers` config so that an Azure provider can be defined as
follows:

```toml
[model_providers.azure]
name = "Azure"
base_url = "https://YOUR_PROJECT_NAME.openai.azure.com/openai"
env_key = "AZURE_OPENAI_API_KEY"
query_params = { api-version = "2025-04-01-preview" }
```

This PR also updates the docs with this example.

While here, we also update `wire_api` to default to `"chat"`, as that is
likely the common case for someone defining an external provider.

Fixes https://github.com/openai/codex/issues/1365.

codex-rs/core/src/chat_completions.rs

@@ -114,8 +114,7 @@ pub(crate) async fn stream_chat_completions(
         "tools": tools_json,
     });
 
-    let base_url = provider.base_url.trim_end_matches('/');
-    let url = format!("{}/chat/completions", base_url);
+    let url = provider.get_full_url();
 
     debug!(
         "POST to {url}: {}",