feat: read model_provider and model_providers from config.toml (#853)

This is the first step in supporting other model providers in the Rust
CLI. Specifically, this PR adds support for the new entries in `Config`
and `ConfigOverrides` to specify a `ModelProviderInfo`, which is the
basic config needed for an LLM provider. This PR does not get us all the
way there yet because `client.rs` still categorically appends
`/responses` to the URL and expects the endpoint to support the OpenAI
Responses API. Will fix that next!

codex-rs/core/src/codex.rs

@@ -80,6 +80,7 @@ impl Codex {
         let (tx_sub, rx_sub) = async_channel::bounded(64);
         let (tx_event, rx_event) = async_channel::bounded(64);
         let configure_session = Op::ConfigureSession {
+            provider: config.model_provider.clone(),
             model: config.model.clone(),
             instructions: config.instructions.clone(),
             approval_policy: config.approval_policy,
@@ -504,6 +505,7 @@ async fn submission_loop(
                 sess.abort();
             }
             Op::ConfigureSession {
+                provider,
                 model,
                 instructions,
                 approval_policy,
@@ -512,7 +514,7 @@ async fn submission_loop(
                 notify,
                 cwd,
             } => {
-                info!(model, "Configuring session");
+                info!("Configuring session: model={model}; provider={provider:?}");
                 if !cwd.is_absolute() {
                     let message = format!("cwd is not absolute: {cwd:?}");
                     error!(message);
@@ -526,7 +528,7 @@ async fn submission_loop(
                     return;
                 }
 
-                let client = ModelClient::new(model.clone());
+                let client = ModelClient::new(model.clone(), provider.clone());
 
                 // abort any current running session and clone its state
                 let state = match sess.take() {