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/Cargo.toml

@@ -21,6 +21,7 @@ members = [
     "keyring-store",
     "file-search",
     "linux-sandbox",
+    "lmstudio",
     "login",
     "mcp-server",
     "mcp-types",
@@ -70,6 +71,7 @@ codex-file-search = { path = "file-search" }
 codex-git = { path = "utils/git" }
 codex-keyring-store = { path = "keyring-store" }
 codex-linux-sandbox = { path = "linux-sandbox" }
+codex-lmstudio = { path = "lmstudio" }
 codex-login = { path = "login" }
 codex-mcp-server = { path = "mcp-server" }
 codex-ollama = { path = "ollama" }