feat: add ZDR support to Rust implementation (#642)

This adds support for the `--disable-response-storage` flag across our
multiple Rust CLIs to support customers who have opted into Zero-Data
Retention (ZDR). The analogous changes to the TypeScript CLI were:

* https://github.com/openai/codex/pull/481
* https://github.com/openai/codex/pull/543

For a client using ZDR, `previous_response_id` will never be available,
so the `input` field of an API request must include the full transcript
of the conversation thus far. As such, this PR changes the type of
`Prompt.input` from `Vec<ResponseInputItem>` to `Vec<ResponseItem>`.

Practically speaking, `ResponseItem` was effectively a "superset" of
`ResponseInputItem` already. The main difference for us is that
`ResponseItem` includes the `FunctionCall` variant that we have to
include as part of the conversation history in the ZDR case.

Another key change in this PR is modifying `try_run_turn()` so that it
returns the `Vec<ResponseItem>` for the turn in addition to the
`Vec<ResponseInputItem>` produced by `try_run_turn()`. This is because
the caller of `run_turn()` needs to record the `Vec<ResponseItem>` when
ZDR is enabled.

To that end, this PR introduces `ZdrTranscript` (and adds
`zdr_transcript: Option<ZdrTranscript>` to `struct State` in `codex.rs`)
to take responsibility for maintaining the conversation transcript in
the ZDR case.

codex-rs/core/src/zdr_transcript.rs

@@ -0,0 +1,46 @@
+use crate::models::ResponseItem;
+
+/// Transcript that needs to be maintained for ZDR clients for which
+/// previous_response_id is not available, so we must include the transcript
+/// with every API call. This must include each `function_call` and its
+/// corresponding `function_call_output`.
+#[derive(Debug, Clone)]
+pub(crate) struct ZdrTranscript {
+    /// The oldest items are at the beginning of the vector.
+    items: Vec<ResponseItem>,
+}
+
+impl ZdrTranscript {
+    pub(crate) fn new() -> Self {
+        Self { items: Vec::new() }
+    }
+
+    /// Returns a clone of the contents in the transcript.
+    pub(crate) fn contents(&self) -> Vec<ResponseItem> {
+        self.items.clone()
+    }
+
+    /// `items` is ordered from oldest to newest.
+    pub(crate) fn record_items<I>(&mut self, items: I)
+    where
+        I: IntoIterator<Item = ResponseItem>,
+    {
+        for item in items {
+            if is_api_message(&item) {
+                // Note agent-loop.ts also does filtering on some of the fields.
+                self.items.push(item.clone());
+            }
+        }
+    }
+}
+
+/// Anything that is not a system message or "reasoning" message is considered
+/// an API message.
+fn is_api_message(message: &ResponseItem) -> bool {
+    match message {
+        ResponseItem::Message { role, .. } => role.as_str() != "system",
+        ResponseItem::FunctionCall { .. } => true,
+        ResponseItem::FunctionCallOutput { .. } => true,
+        _ => false,
+    }
+}