Add per-login MCP OAuth client registration selection (#38052)

## What changed

- Add `--oauth-client-registration` with `auto` and `dcr` choices to `codex mcp add` and `codex mcp login`.
- Add the equivalent optional `clientRegistration` field to `mcpServer/oauth/login` and the generated protocol schemas.
- Keep automatic dynamic client registration as the default, prefer a configured OAuth client ID when present, and apply the selection only to the current login without persisting it in MCP configuration.

## Testing

- Cover dynamic registration and configured-client OAuth flows through the app server.
- Verify the CLI does not persist the per-login registration choice.

GitOrigin-RevId: 653a4a1054ff43c2c5d52ae863b6a96d21692a53

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -2195,6 +2195,7 @@ mod tests {
             params: v2::McpServerOauthLoginParams {
                 name: "server-a".to_string(),
                 thread_id: None,
+                client_registration: None,
                 scopes: None,
                 timeout_secs: None,
             },

codex-rs/app-server-protocol/src/protocol/v2/mcp.rs

@@ -196,6 +196,9 @@ pub struct McpServerOauthLoginParams {
     pub name: String,
     #[ts(optional = nullable)]
     pub thread_id: Option<String>,
+    /// Registration strategy for this login only; omission preserves automatic DCR.
+    #[ts(optional = nullable)]
+    pub client_registration: Option<McpServerOauthClientRegistration>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional = nullable)]
     pub scopes: Option<Vec<String>>,
@@ -204,6 +207,15 @@ pub struct McpServerOauthLoginParams {
     pub timeout_secs: Option<i64>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, Default, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase", export_to = "v2/")]
+pub enum McpServerOauthClientRegistration {
+    #[default]
+    Auto,
+    Dcr,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]