Route MCP servers through explicit environments (#23583)

## Summary
- route each configured MCP server through an explicit per-server
`environment_id` instead of a manager-wide remote toggle
- default omitted `environment_id` to `local`, resolve named ids through
`EnvironmentManager`, and fail only the affected MCP server when an
explicit id is unknown
- keep local stdio on the existing local launcher path for now, while
named-environment stdio uses the selected environment backend and
requires an absolute `cwd`
- allow local HTTP MCP servers to keep using the ambient HTTP client
when no local `Environment` is configured; named-environment HTTP MCPs
use that environment's HTTP client

## Validation
- devbox Bazel build: `bazel build --bes_backend= --bes_results_url=
//codex-rs/cli:codex //codex-rs/rmcp-client:test_stdio_server
//codex-rs/rmcp-client:test_streamable_http_server`
- devbox app-server config matrix with real `config.toml` /
`environments.toml` files covering omitted local, explicit local,
omitted local under remote default, explicit remote stdio, local HTTP
without local env, explicit remote HTTP, local stdio without local env,
unknown explicit env, and remote stdio without `cwd`

codex-rs/core/src/connectors.rs

@@ -35,7 +35,7 @@ use codex_login::CodexAuth;
 use codex_login::default_client::originator;
 use codex_mcp::CODEX_APPS_MCP_SERVER_NAME;
 use codex_mcp::McpConnectionManager;
-use codex_mcp::McpRuntimeEnvironment;
+use codex_mcp::McpRuntimeContext;
 use codex_mcp::ToolInfo;
 use codex_mcp::ToolPluginProvenance;
 use codex_mcp::codex_apps_tools_cache_key;
@@ -203,7 +203,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_options_and_status(
     list_accessible_connectors_from_mcp_tools_with_environment_manager(
         config,
         force_refetch,
-        &environment_manager,
+        Arc::new(environment_manager),
     )
     .await
 }
@@ -211,7 +211,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_options_and_status(
 pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
     config: &Config,
     force_refetch: bool,
-    environment_manager: &EnvironmentManager,
+    environment_manager: Arc<EnvironmentManager>,
 ) -> anyhow::Result<AccessibleConnectorsStatus> {
     let auth_manager =
         AuthManager::shared_from_config(config, /*enable_codex_api_key_env*/ false).await;
@@ -272,11 +272,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
         PermissionProfile::default(),
         // Connector discovery is threadless. Use an actually configured env if
         // one exists, but do not reintroduce the old hidden-local fallback.
-        McpRuntimeEnvironment::new(
-            environment_manager.default_or_local_environment(),
-            environment_manager.try_local_environment(),
-            config.cwd.to_path_buf(),
-        ),
+        McpRuntimeContext::new(environment_manager, config.cwd.to_path_buf()),
         config.codex_home.to_path_buf(),
         codex_apps_tools_cache_key(auth.as_ref()),
         host_owned_codex_apps_enabled,