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/common/src/lib.rs

@@ -37,3 +37,5 @@ pub mod model_presets;
 // Shared approval presets (AskForApproval + Sandbox) used by TUI and MCP server
 // Not to be confused with AskForApproval, which we should probably rename to EscalationPolicy.
 pub mod approval_presets;
+// Shared OSS provider utilities used by TUI and exec
+pub mod oss;

codex-rs/common/src/oss.rs

@@ -0,0 +1,60 @@
+//! OSS provider utilities shared between TUI and exec.
+
+use codex_core::LMSTUDIO_OSS_PROVIDER_ID;
+use codex_core::OLLAMA_OSS_PROVIDER_ID;
+use codex_core::config::Config;
+
+/// Returns the default model for a given OSS provider.
+pub fn get_default_model_for_oss_provider(provider_id: &str) -> Option<&'static str> {
+    match provider_id {
+        LMSTUDIO_OSS_PROVIDER_ID => Some(codex_lmstudio::DEFAULT_OSS_MODEL),
+        OLLAMA_OSS_PROVIDER_ID => Some(codex_ollama::DEFAULT_OSS_MODEL),
+        _ => None,
+    }
+}
+
+/// Ensures the specified OSS provider is ready (models downloaded, service reachable).
+pub async fn ensure_oss_provider_ready(
+    provider_id: &str,
+    config: &Config,
+) -> Result<(), std::io::Error> {
+    match provider_id {
+        LMSTUDIO_OSS_PROVIDER_ID => {
+            codex_lmstudio::ensure_oss_ready(config)
+                .await
+                .map_err(|e| std::io::Error::other(format!("OSS setup failed: {e}")))?;
+        }
+        OLLAMA_OSS_PROVIDER_ID => {
+            codex_ollama::ensure_oss_ready(config)
+                .await
+                .map_err(|e| std::io::Error::other(format!("OSS setup failed: {e}")))?;
+        }
+        _ => {
+            // Unknown provider, skip setup
+        }
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_get_default_model_for_provider_lmstudio() {
+        let result = get_default_model_for_oss_provider(LMSTUDIO_OSS_PROVIDER_ID);
+        assert_eq!(result, Some(codex_lmstudio::DEFAULT_OSS_MODEL));
+    }
+
+    #[test]
+    fn test_get_default_model_for_provider_ollama() {
+        let result = get_default_model_for_oss_provider(OLLAMA_OSS_PROVIDER_ID);
+        assert_eq!(result, Some(codex_ollama::DEFAULT_OSS_MODEL));
+    }
+
+    #[test]
+    fn test_get_default_model_for_provider_unknown() {
+        let result = get_default_model_for_oss_provider("unknown-provider");
+        assert_eq!(result, None);
+    }
+}