Nicer highlighting of slash commands, /plan accepts prompt args and pasted images (#10269)

## Summary
- Make typed slash commands become text elements when the user hits
space, including paste‑burst spaces.
- Enable `/plan` to accept inline args and submit them in plan mode,
mirroring `/review` behavior and blocking submission while a task is
running.
- Preserve text elements/attachments for slash commands that take args.

<img width="1510" height="500" alt="image"
src="https://github.com/user-attachments/assets/446024df-b69a-4249-85db-1a85110e07f1"
/>

## Changes
- Add safe helper to insert element ranges in the textarea.
- Extend command‑with‑args pipeline to carry text elements and reuse
submission prep.
- Update `/plan` dispatch to switch to plan mode then submit prompt +
elements.
- Document new composer behavior and add tests.

## Notes
- `/plan` is blocked during active tasks (same as `/review`).
- Slash‑command elementization recognizes built‑ins and `/prompts:`
custom commands only.

## Codex author
`codex fork 019c16d3-4520-7bb0-9b9d-48720d40a8ab`

codex-rs/tui/src/slash_command.rs

@@ -87,6 +87,14 @@ impl SlashCommand {
         self.into()
     }
 
+    /// Whether this command supports inline args (for example `/review ...`).
+    pub fn supports_inline_args(self) -> bool {
+        matches!(
+            self,
+            SlashCommand::Review | SlashCommand::Rename | SlashCommand::Plan
+        )
+    }
+
     /// Whether this command can be run while a task is in progress.
     pub fn available_during_task(self) -> bool {
         match self {
@@ -103,6 +111,7 @@ impl SlashCommand {
             | SlashCommand::ElevateSandbox
             | SlashCommand::Experimental
             | SlashCommand::Review
+            | SlashCommand::Plan
             | SlashCommand::Logout => false,
             SlashCommand::Diff
             | SlashCommand::Rename
@@ -117,7 +126,6 @@ impl SlashCommand {
             | SlashCommand::Exit => true,
             SlashCommand::Rollout => true,
             SlashCommand::TestApproval => true,
-            SlashCommand::Plan => true,
             SlashCommand::Collab => true,
             SlashCommand::Agent => true,
         }