Finish moving codex exec to app-server (#15424)

This PR completes the conversion of non-interactive `codex exec` to use
app server rather than directly using core events and methods.

### Summary
- move `codex-exec` off exec-owned `AuthManager` and `ThreadManager`
state
- route exec bootstrap, resume, and auth refresh through existing
app-server paths
- replace legacy `codex/event/*` decoding in exec with typed app-server
notification handling
- update human and JSONL exec output adapters to translate existing
app-server notifications only
- clean up "app server client" layer by eliminating support for legacy
notifications; this is no longer needed
- remove exposure of `authManager` and `threadManager` from "app server
client" layer

### Testing
- `exec` has pretty extensive unit and integration tests already, and
these all pass
- In addition, I asked Codex to put together a comprehensive manual set
of tests to cover all of the `codex exec` functionality (including
command-line options), and it successfully generated and ran these tests

codex-rs/tui/tests/suite/model_availability_nux.rs

@@ -178,10 +178,11 @@ trust_level = "trusted"
             anyhow::bail!("timed out waiting for codex resume to exit");
         }
     };
+    let output_text = String::from_utf8_lossy(&output);
+    let interrupted_startup = exit_code == 1 && output_text.trim() == "^C";
     anyhow::ensure!(
-        exit_code == 0 || exit_code == 130,
-        "unexpected exit code from codex resume: {exit_code}; output: {}",
-        String::from_utf8_lossy(&output)
+        exit_code == 0 || exit_code == 130 || interrupted_startup,
+        "unexpected exit code from codex resume: {exit_code}; output: {output_text}",
     );
 
     let config_contents = std::fs::read_to_string(codex_home.path().join("config.toml"))?;