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/tests/live_agent.rs

@@ -55,6 +55,7 @@ async fn spawn_codex() -> Codex {
                 instructions: None,
                 approval_policy: AskForApproval::OnFailure,
                 sandbox_policy: SandboxPolicy::NetworkAndFileWriteRestricted,
+                disable_response_storage: false,
             },
         })
         .await

codex-rs/core/tests/previous_response_id.rs

@@ -95,6 +95,7 @@ async fn keeps_previous_response_id_between_tasks() {
                 instructions: None,
                 approval_policy: AskForApproval::OnFailure,
                 sandbox_policy: SandboxPolicy::NetworkAndFileWriteRestricted,
+                disable_response_storage: false,
             },
         })
         .await

codex-rs/core/tests/stream_no_completed.rs

@@ -78,6 +78,7 @@ async fn retries_on_early_close() {
                 instructions: None,
                 approval_policy: AskForApproval::OnFailure,
                 sandbox_policy: SandboxPolicy::NetworkAndFileWriteRestricted,
+                disable_response_storage: false,
             },
         })
         .await