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/src/client_common.rs

@@ -255,6 +255,7 @@ mod tests {
             stream: true,
             include: vec![],
             prompt_cache_key: None,
+            service_tier: None,
             text: Some(TextControls {
                 verbosity: Some(OpenAiVerbosity::Low),
                 format: None,
@@ -296,6 +297,7 @@ mod tests {
             stream: true,
             include: vec![],
             prompt_cache_key: None,
+            service_tier: None,
             text: Some(text_controls),
         };
 
@@ -332,6 +334,7 @@ mod tests {
             stream: true,
             include: vec![],
             prompt_cache_key: None,
+            service_tier: None,
             text: None,
         };