Prewarm code-mode host connections at session startup (#40678)

## What changed

- Add the opt-in `code_mode_prewarm` feature to establish the code-mode host session during startup, before the first turn.
- Make in-progress host initialization cancellable so a stalled prewarm does not block shutdown.
- Share a failed connection attempt with concurrent callers instead of immediately starting another attempt.

## Testing

- Verify app-server contacts the configured host before the first turn and can shut down while that connection is stalled.
- Exercise shared remote-host behavior with prewarming enabled.

GitOrigin-RevId: 3cfde5509be7b4aba80112c36c5616b9a9b632c6

codex-rs/core/config.schema.json

@@ -628,6 +628,9 @@
             "code_mode_only": {
               "type": "boolean"
             },
+            "code_mode_prewarm": {
+              "type": "boolean"
+            },
             "codex_git_commit": {
               "type": "boolean"
             },
@@ -5640,6 +5643,9 @@
         "code_mode_only": {
           "type": "boolean"
         },
+        "code_mode_prewarm": {
+          "type": "boolean"
+        },
         "codex_git_commit": {
           "type": "boolean"
         },