Make local environment optional in EnvironmentManager (#23369)

## Summary
- make `EnvironmentManager` local environment/runtime paths optional
- simplify constructor surface around snapshot materialization
- rename local env accessors to `require_local_environment` /
`try_local_environment`

## Validation
- devbox Bazel build for touched crate surfaces
- `//codex-rs/exec-server:exec-server-unit-tests`
- `//codex-rs/app-server-client:app-server-client-unit-tests`
- filtered touched `//codex-rs/core:core-unit-tests` cases

codex-rs/codex-mcp/src/connection_manager_tests.rs

@@ -847,6 +847,112 @@ async fn list_all_tools_adds_server_metadata_to_cached_tools() {
     assert_eq!(tool.server_origin.as_deref(), Some("https://docs.example"));
 }
 
+#[tokio::test]
+async fn no_local_runtime_fails_local_stdio_but_keeps_local_http_server() {
+    let approval_policy = Constrained::allow_any(AskForApproval::OnFailure);
+    let (tx_event, rx_event) = async_channel::unbounded();
+    drop(rx_event);
+    let codex_home = tempdir().expect("tempdir");
+    let mcp_servers = HashMap::from([
+        (
+            "stdio".to_string(),
+            EffectiveMcpServer::configured(McpServerConfig {
+                transport: McpServerTransportConfig::Stdio {
+                    command: "echo".to_string(),
+                    args: Vec::new(),
+                    env: None,
+                    env_vars: Vec::new(),
+                    cwd: None,
+                },
+                experimental_environment: None,
+                enabled: true,
+                required: false,
+                supports_parallel_tool_calls: false,
+                disabled_reason: None,
+                startup_timeout_sec: None,
+                tool_timeout_sec: None,
+                default_tools_approval_mode: None,
+                enabled_tools: None,
+                disabled_tools: None,
+                scopes: None,
+                oauth: None,
+                oauth_resource: None,
+                tools: HashMap::new(),
+            }),
+        ),
+        (
+            "http".to_string(),
+            EffectiveMcpServer::configured(McpServerConfig {
+                transport: McpServerTransportConfig::StreamableHttp {
+                    url: "http://127.0.0.1:1".to_string(),
+                    bearer_token_env_var: None,
+                    http_headers: None,
+                    env_http_headers: None,
+                },
+                experimental_environment: None,
+                enabled: true,
+                required: false,
+                supports_parallel_tool_calls: false,
+                disabled_reason: None,
+                startup_timeout_sec: None,
+                tool_timeout_sec: None,
+                default_tools_approval_mode: None,
+                enabled_tools: None,
+                disabled_tools: None,
+                scopes: None,
+                oauth: None,
+                oauth_resource: None,
+                tools: HashMap::new(),
+            }),
+        ),
+    ]);
+
+    let (manager, cancel_token) = McpConnectionManager::new(
+        &mcp_servers,
+        OAuthCredentialsStoreMode::default(),
+        HashMap::new(),
+        &approval_policy,
+        String::new(),
+        tx_event,
+        PermissionProfile::default(),
+        McpRuntimeEnvironment::new(
+            /*environment*/ None,
+            /*local_environment*/ None,
+            PathBuf::from("/tmp"),
+        ),
+        codex_home.path().to_path_buf(),
+        CodexAppsToolsCacheKey {
+            account_id: None,
+            chatgpt_user_id: None,
+            is_workspace_account: false,
+        },
+        /*host_owned_codex_apps_enabled*/ false,
+        ElicitationCapability::default(),
+        ToolPluginProvenance::default(),
+        /*auth*/ None,
+        /*elicitation_reviewer*/ None,
+    )
+    .await;
+
+    assert!(manager.clients.contains_key("stdio"));
+    assert!(manager.clients.contains_key("http"));
+    assert!(
+        !manager
+            .wait_for_server_ready("stdio", Duration::from_millis(10))
+            .await
+    );
+    let failures = manager
+        .required_startup_failures(&["stdio".to_string()])
+        .await;
+    assert_eq!(failures.len(), 1);
+    assert_eq!(failures[0].server, "stdio");
+    assert_eq!(
+        failures[0].error,
+        "local stdio MCP server `stdio` requires a local environment"
+    );
+    cancel_token.cancel();
+}
+
 #[test]
 fn elicitation_capability_uses_2025_06_18_shape_for_form_only_support() {
     let capability = Some(ElicitationCapability::default());