[codex] Add tmux-aware OSC 9 notifications (#17836)

## Summary
- wrap OSC 9 notifications in tmux's DCS passthrough so terminal
notifications make it through tmux
- use codex-terminal-detection for OSC 9 auto-selection so tmux sessions
inherit the underlying client terminal support
- add focused notification backend tests for plain OSC 9 and
tmux-wrapped output

## Stack
- base PR: #18479
- review order: #18479, then this PR

## Why
Tmux does not forward OSC 9 notifications directly; the sequence has to
be wrapped in tmux's DCS passthrough envelope. Codex also had local
notification heuristics that could miss supported terminals when running
under tmux, even though codex-terminal-detection already knows how to
attribute tmux sessions to the client terminal.

## Validation
- `just fmt`
- `cargo test -p codex-tui` *(currently blocked by an unrelated existing
compile error in `app-server/src/message_processor.rs:754` referencing
`connection_id` out of scope; not caused by this change)*

Co-authored-by: Codex <noreply@openai.com>

codex-rs/terminal-detection/src/terminal_tests.rs

@@ -456,6 +456,44 @@ fn detects_wezterm() {
         "WezTerm",
         "wezterm_empty_user_agent"
     );
+
+    let env = FakeEnvironment::new().with_var("TERM", "wezterm");
+    let terminal = detect_terminal_info_from_env(&env);
+    assert_eq!(
+        terminal,
+        terminal_info(
+            TerminalName::WezTerm,
+            /*term_program*/ None,
+            /*version*/ None,
+            Some("wezterm"),
+            /*multiplexer*/ None
+        ),
+        "wezterm_term_info"
+    );
+    assert_eq!(
+        terminal.user_agent_token(),
+        "wezterm",
+        "wezterm_term_user_agent"
+    );
+
+    let env = FakeEnvironment::new().with_var("TERM", "wezterm-mux");
+    let terminal = detect_terminal_info_from_env(&env);
+    assert_eq!(
+        terminal,
+        terminal_info(
+            TerminalName::WezTerm,
+            /*term_program*/ None,
+            /*version*/ None,
+            Some("wezterm-mux"),
+            /*multiplexer*/ None
+        ),
+        "wezterm_mux_term_info"
+    );
+    assert_eq!(
+        terminal.user_agent_token(),
+        "wezterm-mux",
+        "wezterm_mux_term_user_agent"
+    );
 }
 
 #[test]