app-server service tier plumbing (plus some cleanup) (#13334)

followup to https://github.com/openai/codex/pull/13212 to expose fast
tier controls to app server
(majority of this PR is generated schema jsons - actual code is +69 /
-35 and +24 tests )

- add service tier fields to the app-server protocol surfaces used by
thread lifecycle, turn start, config, and session configured events
- thread service tier through the app-server message processor and core
thread config snapshots
- allow runtime config overrides to carry service tier for app-server
callers

cleanup:
- Removing useless "legacy" code supporting "standard" - we moved to
None | "fast", so "standard" is not needed.

codex-rs/app-server-protocol/src/protocol/serde_helpers.rs

@@ -0,0 +1,23 @@
+use serde::Deserialize;
+use serde::Deserializer;
+use serde::Serialize;
+use serde::Serializer;
+
+pub fn deserialize_double_option<'de, T, D>(deserializer: D) -> Result<Option<Option<T>>, D::Error>
+where
+    T: Deserialize<'de>,
+    D: Deserializer<'de>,
+{
+    serde_with::rust::double_option::deserialize(deserializer)
+}
+
+pub fn serialize_double_option<T, S>(
+    value: &Option<Option<T>>,
+    serializer: S,
+) -> Result<S::Ok, S::Error>
+where
+    T: Serialize,
+    S: Serializer,
+{
+    serde_with::rust::double_option::serialize(value, serializer)
+}