Support Codex CLI stdin piping for codex exec (#15917)

# Summary

Claude Code supports a useful prompt-plus-stdin workflow:

```bash
echo "complex input..." | claude -p "summarize concisely"
```

Codex previously did not support the equivalent `codex exec` form. While
`codex exec` could read the prompt from stdin, it could not combine
piped input with an explicit prompt argument.

This change adds that missing workflow:

```bash
echo "complex input..." | codex exec "summarize concisely"
```

With this change, when `codex exec` receives both a positional prompt
and piped stdin, the prompt remains the instruction and stdin is passed
along as structured `<stdin>...</stdin>` context.

Example:

```bash
curl https://jsonplaceholder.typicode.com/comments \
  | ./target/debug/codex exec --skip-git-repo-check "format the top 20 items into a markdown table" \
  > table.md
```

This PR also adds regression coverage for:
- prompt argument + piped stdin
- legacy stdin-as-prompt behavior
- `codex exec -` forced-stdin behavior
- empty-stdin error cases

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec/src/cli.rs

@@ -105,7 +105,8 @@ pub struct Cli {
     pub last_message_file: Option<PathBuf>,
 
     /// Initial instructions for the agent. If not provided as an argument (or
-    /// if `-` is used), instructions are read from stdin.
+    /// if `-` is used), instructions are read from stdin. If stdin is piped and
+    /// a prompt is also provided, stdin is appended as a `<stdin>` block.
     #[arg(value_name = "PROMPT", value_hint = clap::ValueHint::Other)]
     pub prompt: Option<String>,
 }