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/exec/tests/suite/output_schema.rs

@@ -37,7 +37,7 @@ async fn exec_includes_output_schema_in_request() -> anyhow::Result<()> {
         .arg("--output-schema")
         .arg(&schema_path)
         .arg("-m")
-        .arg("gpt-5")
+        .arg("gpt-5.1")
         .arg("tell me a joke")
         .assert()
         .success();

codex-rs/exec/tests/suite/resume.rs

@@ -196,7 +196,7 @@ fn exec_resume_preserves_cli_configuration_overrides() -> anyhow::Result<()> {
         .arg("--sandbox")
         .arg("workspace-write")
         .arg("--model")
-        .arg("gpt-5")
+        .arg("gpt-5.1")
         .arg("-C")
         .arg(env!("CARGO_MANIFEST_DIR"))
         .arg(&prompt)
@@ -218,7 +218,7 @@ fn exec_resume_preserves_cli_configuration_overrides() -> anyhow::Result<()> {
         .arg("--sandbox")
         .arg("workspace-write")
         .arg("--model")
-        .arg("gpt-5-high")
+        .arg("gpt-5.1-high")
         .arg("-C")
         .arg(env!("CARGO_MANIFEST_DIR"))
         .arg(&prompt2)
@@ -231,7 +231,7 @@ fn exec_resume_preserves_cli_configuration_overrides() -> anyhow::Result<()> {
 
     let stderr = String::from_utf8(output.stderr)?;
     assert!(
-        stderr.contains("model: gpt-5-high"),
+        stderr.contains("model: gpt-5.1-high"),
         "stderr missing model override: {stderr}"
     );
     if cfg!(target_os = "windows") {