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/codex-mcp/src/connection_manager_tests.rs

@@ -18,6 +18,7 @@ use crate::tools::normalize_tools_for_model;
 use crate::tools::tool_with_model_visible_input_schema;
 use codex_config::Constrained;
 use codex_config::McpServerConfig;
+use codex_exec_server::EnvironmentManager;
 use codex_protocol::ToolName;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::protocol::GranularApprovalConfig;
@@ -864,7 +865,7 @@ async fn no_local_runtime_fails_local_stdio_but_keeps_local_http_server() {
                     env_vars: Vec::new(),
                     cwd: None,
                 },
-                experimental_environment: None,
+                environment_id: codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID.to_string(),
                 enabled: true,
                 required: false,
                 supports_parallel_tool_calls: false,
@@ -889,7 +890,7 @@ async fn no_local_runtime_fails_local_stdio_but_keeps_local_http_server() {
                     http_headers: None,
                     env_http_headers: None,
                 },
-                experimental_environment: None,
+                environment_id: codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID.to_string(),
                 enabled: true,
                 required: false,
                 supports_parallel_tool_calls: false,
@@ -915,9 +916,8 @@ async fn no_local_runtime_fails_local_stdio_but_keeps_local_http_server() {
         String::new(),
         tx_event,
         PermissionProfile::default(),
-        McpRuntimeEnvironment::new(
-            /*environment*/ None,
-            /*local_environment*/ None,
+        McpRuntimeContext::new(
+            Arc::new(EnvironmentManager::without_environments()),
             PathBuf::from("/tmp"),
         ),
         codex_home.path().to_path_buf(),
@@ -988,7 +988,7 @@ fn mcp_init_error_display_prompts_for_github_pat() {
                 http_headers: None,
                 env_http_headers: None,
             },
-            experimental_environment: None,
+            environment_id: codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID.to_string(),
             enabled: true,
             required: false,
             supports_parallel_tool_calls: false,
@@ -1041,7 +1041,7 @@ fn mcp_init_error_display_reports_generic_errors() {
                 http_headers: None,
                 env_http_headers: None,
             },
-            experimental_environment: None,
+            environment_id: codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID.to_string(),
             enabled: true,
             required: false,
             supports_parallel_tool_calls: false,