Remove async-trait from extension contributors (#27383)

## Why

Extension contributors are registered behind `dyn Trait` objects, so
native `async fn`/RPITIT methods would make these traits
non-object-safe. Spell out the boxed, `Send` future contract directly so
`extension-api` no longer needs `async-trait` while retaining the
existing runtime model.

## What changed

- add a shared `ExtensionFuture` alias and use it for asynchronous
contributor methods
- migrate production and test implementations to return `Box::pin(async
move { ... })`
- remove `async-trait` dependencies where they are no longer used,
keeping it dev-only where unrelated test executors still require it

## Behavior

No behavior change is intended. Contributor futures remain boxed,
`Send`, dynamically dispatched, and lazily executed; cancellation and
callback ordering stay unchanged.

## Testing

- `just test -p codex-extension-api` (11 passed)
- affected extension crates (64 passed)
- targeted `codex-core` contributor tests (14 passed)
- `just fmt`
- `just bazel-lock-update`
- `just bazel-lock-check`

A broad local `codex-core` run compiled successfully but encountered
unrelated sandbox and missing test-binary fixture failures; CI will run
the full checks.

codex-rs/ext/mcp/src/lib.rs

@@ -1,4 +1,5 @@
 use codex_core::config::Config;
+use codex_extension_api::ExtensionFuture;
 use codex_extension_api::ExtensionRegistryBuilder;
 use codex_extension_api::McpServerContribution;
 use codex_extension_api::McpServerContributor;
@@ -7,21 +8,25 @@ use codex_mcp::hosted_plugin_runtime_mcp_server_config;
 
 struct HostedPluginRuntimeExtension;
 
-#[async_trait::async_trait]
 impl McpServerContributor<Config> for HostedPluginRuntimeExtension {
-    async fn contribute(&self, config: &Config) -> Vec<McpServerContribution> {
-        let name = CODEX_APPS_MCP_SERVER_NAME.to_string();
-        if !config.features.enabled(codex_features::Feature::Apps) {
-            return vec![McpServerContribution::Remove { name }];
-        }
+    fn contribute<'a>(
+        &'a self,
+        config: &'a Config,
+    ) -> ExtensionFuture<'a, Vec<McpServerContribution>> {
+        Box::pin(async move {
+            let name = CODEX_APPS_MCP_SERVER_NAME.to_string();
+            if !config.features.enabled(codex_features::Feature::Apps) {
+                return vec![McpServerContribution::Remove { name }];
+            }
 
-        vec![McpServerContribution::Set {
-            name,
-            config: Box::new(hosted_plugin_runtime_mcp_server_config(
-                &config.chatgpt_base_url,
-                config.apps_mcp_product_sku.as_deref(),
-            )),
-        }]
+            vec![McpServerContribution::Set {
+                name,
+                config: Box::new(hosted_plugin_runtime_mcp_server_config(
+                    &config.chatgpt_base_url,
+                    config.apps_mcp_product_sku.as_deref(),
+                )),
+            }]
+        })
     }
 }