[app-server] small fixes for JSON schema export and one-of types (#6614)

A partner is consuming our generated JSON schema bundle for app-server
and identified a few issues:
- not all polymorphic / one-of types have a type descriminator
- `"$ref": "#/definitions/v2/SandboxPolicy"` is missing
- "Option<>" is an invalid schema name, and also unnecessary

This PR:
- adds the type descriminator to the various types that are missing it
except for `SessionSource` and `SubAgentSource` because they are
serialized to disk (adding this would break backwards compat for
resume), and they should not be necessary to consume for an integration
with app-server.
- removes the special handling in `export.rs` of various types like
SandboxPolicy, which turned out to be unnecessary and incorrect
- filters out `Option<>` which was auto-generated for request params
that don't need a body

For context, we currently pull in wayyy more types than we need through
the `EventMsg` god object which we are **not** planning to expose in API
v2 (this is how I suspect `SessionSource` and `SubAgentSource` are being
pulled in). But until we have all the necessary v2 notifications in
place that will allow us to remove `EventMsg`, we will keep exporting it
for now.

codex-rs/protocol/src/items.rs

@@ -11,6 +11,8 @@ use serde::Serialize;
 use ts_rs::TS;
 
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
+#[serde(tag = "type")]
+#[ts(tag = "type")]
 pub enum TurnItem {
     UserMessage(UserMessageItem),
     AgentMessage(AgentMessageItem),
@@ -25,6 +27,8 @@ pub struct UserMessageItem {
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
+#[serde(tag = "type")]
+#[ts(tag = "type")]
 pub enum AgentMessageContent {
     Text { text: String },
 }