Support --output-schema for exec resume (#23123)

## Why

`codex exec resume` should have the same structured-output support as
top-level `codex exec`. Without `--output-schema`, multi-turn automation
has to choose between resumed session context and schema-validated JSON
output.

Fixes #22998.

## What changed

- Marked `--output-schema` as a global `codex exec` flag so it can be
passed after `resume`.
- Reused the existing output schema plumbing so resumed turns attach the
schema to the final response request while preserving session context.

codex-rs/exec/src/cli_tests.rs

@@ -36,7 +36,7 @@ fn resume_parses_prompt_after_global_flags() {
 }
 
 #[test]
-fn resume_accepts_output_last_message_flag_after_subcommand() {
+fn resume_accepts_output_flags_after_subcommand() {
     const PROMPT: &str = "echo resume-with-output-file";
     let cli = Cli::parse_from([
         "codex-exec",
@@ -44,6 +44,8 @@ fn resume_accepts_output_last_message_flag_after_subcommand() {
         "session-123",
         "-o",
         "/tmp/resume-output.md",
+        "--output-schema",
+        "/tmp/schema.json",
         PROMPT,
     ]);
 
@@ -51,6 +53,7 @@ fn resume_accepts_output_last_message_flag_after_subcommand() {
         cli.last_message_file,
         Some(PathBuf::from("/tmp/resume-output.md"))
     );
+    assert_eq!(cli.output_schema, Some(PathBuf::from("/tmp/schema.json")));
     let Some(Command::Resume(args)) = cli.command else {
         panic!("expected resume command");
     };