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/tests/previous_response_id.rs

@@ -107,6 +107,7 @@ async fn keeps_previous_response_id_between_tasks() {
         env_key: Some("PATH".into()),
         env_key_instructions: None,
         wire_api: codex_core::WireApi::Responses,
+        query_params: None,
     };
 
     // Init session

codex-rs/core/tests/stream_no_completed.rs

@@ -96,6 +96,7 @@ async fn retries_on_early_close() {
         env_key: Some("PATH".into()),
         env_key_instructions: None,
         wire_api: codex_core::WireApi::Responses,
+        query_params: None,
     };
 
     let ctrl_c = std::sync::Arc::new(tokio::sync::Notify::new());