TUI config cleanup: oss_provider (#24254)

## Summary

Manual provider selection during `codex --oss` startup was still
persisting `oss_provider` through the legacy local `config.toml` writer.
That bypasses the app-server-owned config mutation path used by the TUI,
so this routes the write through the app server config API instead.

The net behavior is intentionally narrow: only an interactive picker
selection is persisted. Auto-detected single-running-provider startup
and explicit `--local-provider` startup remain ephemeral, so merely
having one backend running does not make that provider sticky for future
runs.

## What Changed

- Removed the TUI picker’s direct dependency on
`set_default_oss_provider`.
- Had `oss_selection` report whether the returned provider came from the
interactive picker.
- Carried only manually selected providers into startup persistence.
- Wrote `oss_provider` via `config/batchWrite` once the app server
session is available.
- Logged a warning and continued startup if the app-server config write
fails.

## Verification

Manually smoke-tested the real `codex-tui` binary with a temporary
`CODEX_HOME`, pseudo-terminal input, and a fake LM Studio HTTP server:

- Interactive picker selection persisted `oss_provider = "lmstudio"`.
- Non-picker `--local-provider lmstudio` startup did not persist
`oss_provider`.

codex-rs/tui/src/oss_selection.rs

@@ -4,7 +4,6 @@ use std::sync::LazyLock;
 use crate::key_hint;
 use crate::key_hint::KeyBinding;
 use crate::key_hint::KeyBindingListExt;
-use crate::legacy_core::config::set_default_oss_provider;
 use codex_model_provider_info::DEFAULT_LMSTUDIO_PORT;
 use codex_model_provider_info::DEFAULT_OLLAMA_PORT;
 use codex_model_provider_info::LMSTUDIO_OSS_PROVIDER_ID;
@@ -309,7 +308,12 @@ fn get_status_symbol_and_color(status: &ProviderStatus) -> (&'static str, Color)
     }
 }
 
-pub async fn select_oss_provider(codex_home: &std::path::Path) -> io::Result<String> {
+pub(crate) struct OssProviderSelection {
+    pub(crate) provider: String,
+    pub(crate) manually_selected: bool,
+}
+
+pub async fn select_oss_provider() -> io::Result<OssProviderSelection> {
     // Check provider statuses first
     let lmstudio_status = check_lmstudio_status().await;
     let ollama_status = check_ollama_status().await;
@@ -318,11 +322,17 @@ pub async fn select_oss_provider(codex_home: &std::path::Path) -> io::Result<Str
     match (&lmstudio_status, &ollama_status) {
         (ProviderStatus::Running, ProviderStatus::NotRunning) => {
             let provider = LMSTUDIO_OSS_PROVIDER_ID.to_string();
-            return Ok(provider);
+            return Ok(OssProviderSelection {
+                provider,
+                manually_selected: false,
+            });
         }
         (ProviderStatus::NotRunning, ProviderStatus::Running) => {
             let provider = OLLAMA_OSS_PROVIDER_ID.to_string();
-            return Ok(provider);
+            return Ok(OssProviderSelection {
+                provider,
+                manually_selected: false,
+            });
         }
         _ => {
             // Both running or both not running - show UI
@@ -346,21 +356,16 @@ pub async fn select_oss_provider(codex_home: &std::path::Path) -> io::Result<Str
         if let Event::Key(key_event) = event::read()?
             && let Some(selection) = widget.handle_key_event(key_event)
         {
-            break Ok(selection);
+            break Ok(OssProviderSelection {
+                provider: selection,
+                manually_selected: true,
+            });
         }
     };
 
     disable_raw_mode()?;
     execute!(terminal.backend_mut(), LeaveAlternateScreen)?;
 
-    // If the user manually selected an OSS provider, we save it as the
-    // default one to use later.
-    if let Ok(ref provider) = result
-        && let Err(e) = set_default_oss_provider(codex_home, provider)
-    {
-        tracing::warn!("Failed to save OSS provider preference: {e}");
-    }
-
     result
 }