Show OAuth error descriptions in callback responses (#9654)

### Motivation
- The local OAuth callback server returned a generic "Invalid OAuth
callback" on failures even when the query contained an
`error_description`, making it hard to debug OAuth failures.

### Description
- Update `codex-rs/rmcp-client/src/perform_oauth_login.rs` to surface
`error_description` values from the callback query in the HTTP response.
- Introduce a `CallbackOutcome` enum and change `parse_oauth_callback`
to return it, parsing `code`, `state`, and `error_description` from the
query string.
- Change `spawn_callback_server` to match on `CallbackOutcome` and
return `OAuth error: <description>` with a 400 status when
`error_description` is present, while preserving the existing success
and invalid flows.
- Use inline formatting for the error response string.

### Testing
- Ran `just fmt` in the `codex-rs` workspace to format changes
successfully.
- Ran `cargo test -p codex-rmcp-client` and all tests passed.

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6971aadc68d0832e93159efea8cd48a9)

codex-rs/rmcp-client/src/perform_oauth_login.rs

@@ -103,21 +103,32 @@ fn spawn_callback_server(server: Arc<Server>, tx: oneshot::Sender<(String, Strin
     tokio::task::spawn_blocking(move || {
         while let Ok(request) = server.recv() {
             let path = request.url().to_string();
-            if let Some(OauthCallbackResult { code, state }) = parse_oauth_callback(&path) {
-                let response =
-                    Response::from_string("Authentication complete. You may close this window.");
-                if let Err(err) = request.respond(response) {
-                    eprintln!("Failed to respond to OAuth callback: {err}");
+            match parse_oauth_callback(&path) {
+                CallbackOutcome::Success(OauthCallbackResult { code, state }) => {
+                    let response = Response::from_string(
+                        "Authentication complete. You may close this window.",
+                    );
+                    if let Err(err) = request.respond(response) {
+                        eprintln!("Failed to respond to OAuth callback: {err}");
+                    }
+                    if let Err(err) = tx.send((code, state)) {
+                        eprintln!("Failed to send OAuth callback: {err:?}");
+                    }
+                    break;
                 }
-                if let Err(err) = tx.send((code, state)) {
-                    eprintln!("Failed to send OAuth callback: {err:?}");
+                CallbackOutcome::Error(description) => {
+                    let response = Response::from_string(format!("OAuth error: {description}"))
+                        .with_status_code(400);
+                    if let Err(err) = request.respond(response) {
+                        eprintln!("Failed to respond to OAuth callback: {err}");
+                    }
                 }
-                break;
-            } else {
-                let response =
-                    Response::from_string("Invalid OAuth callback").with_status_code(400);
-                if let Err(err) = request.respond(response) {
-                    eprintln!("Failed to respond to OAuth callback: {err}");
+                CallbackOutcome::Invalid => {
+                    let response =
+                        Response::from_string("Invalid OAuth callback").with_status_code(400);
+                    if let Err(err) = request.respond(response) {
+                        eprintln!("Failed to respond to OAuth callback: {err}");
+                    }
                 }
             }
         }
@@ -129,29 +140,49 @@ struct OauthCallbackResult {
     state: String,
 }
 
-fn parse_oauth_callback(path: &str) -> Option<OauthCallbackResult> {
-    let (route, query) = path.split_once('?')?;
+enum CallbackOutcome {
+    Success(OauthCallbackResult),
+    Error(String),
+    Invalid,
+}
+
+fn parse_oauth_callback(path: &str) -> CallbackOutcome {
+    let Some((route, query)) = path.split_once('?') else {
+        return CallbackOutcome::Invalid;
+    };
     if route != "/callback" {
-        return None;
+        return CallbackOutcome::Invalid;
     }
 
     let mut code = None;
     let mut state = None;
+    let mut error_description = None;
 
     for pair in query.split('&') {
-        let (key, value) = pair.split_once('=')?;
-        let decoded = decode(value).ok()?.into_owned();
+        let Some((key, value)) = pair.split_once('=') else {
+            continue;
+        };
+        let Ok(decoded) = decode(value) else {
+            continue;
+        };
+        let decoded = decoded.into_owned();
         match key {
             "code" => code = Some(decoded),
             "state" => state = Some(decoded),
+            "error_description" => error_description = Some(decoded),
             _ => {}
         }
     }
 
-    Some(OauthCallbackResult {
-        code: code?,
-        state: state?,
-    })
+    if let (Some(code), Some(state)) = (code, state) {
+        return CallbackOutcome::Success(OauthCallbackResult { code, state });
+    }
+
+    if let Some(description) = error_description {
+        return CallbackOutcome::Error(description);
+    }
+
+    CallbackOutcome::Invalid
 }
 
 pub struct OauthLoginHandle {