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/client.rs

@@ -123,9 +123,7 @@ impl ModelClient {
             stream: true,
         };
 
-        let base_url = self.provider.base_url.clone();
-        let base_url = base_url.trim_end_matches('/');
-        let url = format!("{}/responses", base_url);
+        let url = self.provider.get_full_url();
         trace!("POST to {url}: {}", serde_json::to_string(&payload)?);
 
         let mut attempt = 0;