Overhaul shell detection and centralize command generation for unified exec (#6577)

This fixes command display for unified exec. All `cd`s and `ls`es are
now parsed.

<img width="452" height="237" alt="image"
src="https://github.com/user-attachments/assets/ce92d81f-f74c-485a-9b34-1eaa29290ec6"
/>

Deletes a ton of tests that were doing nothing from shell.rs.

---------

Co-authored-by: Pavel Krymets <pavel@krymets.com>

codex-rs/core/tests/suite/unified_exec.rs

@@ -214,7 +214,11 @@ async fn unified_exec_emits_exec_command_begin_event() -> Result<()> {
 
     assert_eq!(
         begin_event.command,
-        vec!["/bin/echo hello unified exec".to_string()]
+        vec![
+            "/bin/bash".to_string(),
+            "-lc".to_string(),
+            "/bin/echo hello unified exec".to_string()
+        ]
     );
     assert_eq!(begin_event.cwd, cwd.path());
 
@@ -654,7 +658,14 @@ async fn unified_exec_skips_begin_event_for_empty_input() -> Result<()> {
         "expected only the initial command to emit begin event"
     );
     assert_eq!(begin_events[0].call_id, open_call_id);
-    assert_eq!(begin_events[0].command[0], "/bin/sh -c echo ready");
+    assert_eq!(
+        begin_events[0].command,
+        vec![
+            "/bin/bash".to_string(),
+            "-lc".to_string(),
+            "/bin/sh -c echo ready".to_string()
+        ]
+    );
 
     Ok(())
 }