Update defaults to gpt-5.1 (#6652)

## Summary
- update documentation, example configs, and automation defaults to
reference gpt-5.1 / gpt-5.1-codex
- bump the CLI and core configuration defaults, model presets, and error
messaging to the new models while keeping the model-family/tool coverage
for legacy slugs
- refresh tests, fixtures, and TUI snapshots so they expect the upgraded
defaults

## Testing
- `cargo test -p codex-core
config::tests::test_precedence_fixture_with_gpt5_profile`


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6916c5b3c2b08321ace04ee38604fc6b)

codex-rs/docs/codex_mcp_interface.md

@@ -55,7 +55,7 @@ Start a new session with optional overrides:
 
 Request `newConversation` params (subset):
 
-- `model`: string model id (e.g. "o3", "gpt-5", "gpt-5-codex")
+- `model`: string model id (e.g. "o3", "gpt-5.1", "gpt-5.1-codex")
 - `profile`: optional named profile
 - `cwd`: optional working directory
 - `approvalPolicy`: `untrusted` | `on-request` | `on-failure` | `never`
@@ -119,13 +119,13 @@ For the complete request/response shapes and flow examples, see the [“Auth end
 ## Example: start and send a message
 
 ```json
-{ "jsonrpc": "2.0", "id": 1, "method": "newConversation", "params": { "model": "gpt-5", "approvalPolicy": "on-request" } }
+{ "jsonrpc": "2.0", "id": 1, "method": "newConversation", "params": { "model": "gpt-5.1", "approvalPolicy": "on-request" } }
 ```
 
 Server responds:
 
 ```json
-{ "jsonrpc": "2.0", "id": 1, "result": { "conversationId": "c7b0…", "model": "gpt-5", "rolloutPath": "/path/to/rollout.jsonl" } }
+{ "jsonrpc": "2.0", "id": 1, "result": { "conversationId": "c7b0…", "model": "gpt-5.1", "rolloutPath": "/path/to/rollout.jsonl" } }
 ```
 
 Then send input: