LM Studio OSS Support (#2312)

## Overview

Adds LM Studio OSS support. Closes #1883


### Changes
This PR enhances the behavior of `--oss` flag to support LM Studio as a
provider. Additionally, it introduces a new flag`--local-provider` which
can take in `lmstudio` or `ollama` as values if the user wants to
explicitly choose which one to use.

If no provider is specified `codex --oss` will auto-select the provider
based on whichever is running.

#### Additional enhancements 
The default can be set using `oss-provider` in config like:

```
oss_provider = "lmstudio"
```

For non-interactive users, they will need to either provide the provider
as an arg or have it in their `config.toml`

### Notes
For best performance, [set the default context
length](https://lmstudio.ai/docs/app/advanced/per-model) for gpt-oss to
the maximum your machine can support

---------

Co-authored-by: Matt Clayton <matt@lmstudio.ai>
Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/lmstudio/src/lib.rs

@@ -0,0 +1,43 @@
+mod client;
+
+pub use client::LMStudioClient;
+use codex_core::config::Config;
+
+/// Default OSS model to use when `--oss` is passed without an explicit `-m`.
+pub const DEFAULT_OSS_MODEL: &str = "openai/gpt-oss-20b";
+
+/// Prepare the local OSS environment when `--oss` is selected.
+///
+/// - Ensures a local LM Studio server is reachable.
+/// - Checks if the model exists locally and downloads it if missing.
+pub async fn ensure_oss_ready(config: &Config) -> std::io::Result<()> {
+    let model: &str = config.model.as_ref();
+
+    // Verify local LM Studio is reachable.
+    let lmstudio_client = LMStudioClient::try_from_provider(config).await?;
+
+    match lmstudio_client.fetch_models().await {
+        Ok(models) => {
+            if !models.iter().any(|m| m == model) {
+                lmstudio_client.download_model(model).await?;
+            }
+        }
+        Err(err) => {
+            // Not fatal; higher layers may still proceed and surface errors later.
+            tracing::warn!("Failed to query local models from LM Studio: {}.", err);
+        }
+    }
+
+    // Load the model in the background
+    tokio::spawn({
+        let client = lmstudio_client.clone();
+        let model = model.to_string();
+        async move {
+            if let Err(e) = client.load_model(&model).await {
+                tracing::warn!("Failed to load model {}: {}", model, e);
+            }
+        }
+    });
+
+    Ok(())
+}