[codex-tui] exit when terminal is dumb (#9293)

Using terminal with TERM=dumb specifically mean that TUIs and the like
don't work. Ensure that codex doesn't run in these environments and exit
with odd errors like crossterm's "Error: The cursor position could not
be read within a normal duration"

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/core/src/terminal.rs

@@ -47,6 +47,8 @@ pub enum TerminalName {
     Vte,
     /// Windows Terminal emulator.
     WindowsTerminal,
+    /// Dumb terminal (TERM=dumb).
+    Dumb,
     /// Unknown or missing terminal identification.
     Unknown,
 }
@@ -131,7 +133,12 @@ impl TerminalInfo {
 
     /// Creates terminal metadata from a `TERM` capability value.
     fn from_term(term: String, multiplexer: Option<Multiplexer>) -> Self {
-        Self::new(TerminalName::Unknown, None, None, Some(term), multiplexer)
+        let name = if term == "dumb" {
+            TerminalName::Dumb
+        } else {
+            TerminalName::Unknown
+        };
+        Self::new(name, None, None, Some(term), multiplexer)
     }
 
     /// Creates terminal metadata for unknown terminals.
@@ -166,6 +173,7 @@ impl TerminalInfo {
                 TerminalName::GnomeTerminal => "gnome-terminal".to_string(),
                 TerminalName::Vte => format_terminal_version("VTE", &self.version),
                 TerminalName::WindowsTerminal => "WindowsTerminal".to_string(),
+                TerminalName::Dumb => "dumb".to_string(),
                 TerminalName::Unknown => "unknown".to_string(),
             }
         };
@@ -435,6 +443,7 @@ fn terminal_name_from_term_program(value: &str) -> Option<TerminalName> {
         "gnometerminal" => Some(TerminalName::GnomeTerminal),
         "vte" => Some(TerminalName::Vte),
         "windowsterminal" => Some(TerminalName::WindowsTerminal),
+        "dumb" => Some(TerminalName::Dumb),
         _ => None,
     }
 }
@@ -1136,6 +1145,15 @@ mod tests {
             "term_fallback_user_agent"
         );
 
+        let env = FakeEnvironment::new().with_var("TERM", "dumb");
+        let terminal = detect_terminal_info_from_env(&env);
+        assert_eq!(
+            terminal,
+            terminal_info(TerminalName::Dumb, None, None, Some("dumb"), None),
+            "dumb_term_info"
+        );
+        assert_eq!(terminal.user_agent_token(), "dumb", "dumb_term_user_agent");
+
         let env = FakeEnvironment::new();
         let terminal = detect_terminal_info_from_env(&env);
         assert_eq!(