add fast mode toggle (#13212)

- add a local Fast mode setting in codex-core (similar to how model id
is currently stored on disk locally)
- send `service_tier=priority` on requests when Fast is enabled
- add `/fast` in the TUI and persist it locally
- feature flag

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

@@ -162,6 +162,7 @@ async fn run_snapshot_command_with_options(
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -248,6 +249,7 @@ async fn run_shell_command_snapshot_with_options(
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -317,6 +319,7 @@ async fn run_tool_turn_on_harness(
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -535,6 +538,7 @@ async fn shell_command_snapshot_still_intercepts_apply_patch() -> Result<()> {
             model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })