codex-exec: allow resume --last to read prompt #6717 (#6719)

### Description

- codex exec --json resume --last "<prompt>" bailed out because clap
treated the prompt as SESSION_ID. I removed the conflicts_with flag and
reinterpret that positional as a prompt when
--last is set, so the flow now keeps working in JSON mode.
(codex-rs/exec/src/cli.rs:84-104, codex-rs/exec/src/lib.rs:75-130)
- Added a regression test that exercises resume --last in JSON mode to
ensure the prompt is accepted and the rollout file is updated.
(codex-rs/exec/tests/suite/resume.rs:126-178)

### Testing

  - just fmt
  - cargo test -p codex-exec
  - just fix -p codex-exec
  - cargo test -p codex-exec

#6717

Signed-off-by: Dmitri Khokhlov <dkhokhlov@cribl.io>

codex-rs/exec/src/cli.rs

@@ -101,7 +101,7 @@ pub struct ResumeArgs {
     pub session_id: Option<String>,
 
     /// Resume the most recent recorded session (newest) without specifying an id.
-    #[arg(long = "last", default_value_t = false, conflicts_with = "session_id")]
+    #[arg(long = "last", default_value_t = false)]
     pub last: bool,
 
     /// Prompt to send after resuming the session. If `-` is used, read from stdin.