[codex] Fix plugin marketplace help usage (#18710)

## Summary
- Updates generated CLI help for plugin marketplace commands to show the
full `codex plugin marketplace ...` namespace.
- Adds a regression test covering the marketplace command and its `add`,
`upgrade`, and `remove` help pages.

## Root Cause
The marketplace parser already lived under `codex plugin marketplace`,
but Clap generated usage text from the child parser's standalone command
name. That made help output show stale `codex marketplace ...`
instructions even though the top-level `codex marketplace` command no
longer parses.

## Validation
- `just fmt`
- `cargo test -p codex-cli`
- `./target/debug/codex plugin marketplace --help`

codex-rs/cli/src/marketplace_cmd.rs

@@ -13,6 +13,7 @@ use codex_core::plugins::remove_marketplace;
 use codex_utils_cli::CliConfigOverrides;
 
 #[derive(Debug, Parser)]
+#[command(bin_name = "codex plugin marketplace")]
 pub struct MarketplaceCli {
     #[clap(flatten)]
     pub config_overrides: CliConfigOverrides,
@@ -29,6 +30,7 @@ enum MarketplaceSubcommand {
 }
 
 #[derive(Debug, Parser)]
+#[command(bin_name = "codex plugin marketplace add")]
 struct AddMarketplaceArgs {
     /// Marketplace source. Supports owner/repo[@ref], HTTP(S) Git URLs, SSH URLs,
     /// or local marketplace root directories.
@@ -46,11 +48,13 @@ struct AddMarketplaceArgs {
 }
 
 #[derive(Debug, Parser)]
+#[command(bin_name = "codex plugin marketplace upgrade")]
 struct UpgradeMarketplaceArgs {
     marketplace_name: Option<String>,
 }
 
 #[derive(Debug, Parser)]
+#[command(bin_name = "codex plugin marketplace remove")]
 struct RemoveMarketplaceArgs {
     /// Configured marketplace name to remove.
     marketplace_name: String,