mcp: remove codex/sandbox-state custom request support (#17957)

## Why

#17763 moved sandbox-state delivery for MCP tool calls to request
`_meta` via the `codex/sandbox-state-meta` experimental capability.
Keeping the older `codex/sandbox-state` capability meant Codex still
maintained a second transport that pushed updates with the custom
`codex/sandbox-state/update` request at server startup and when the
session sandbox policy changed.

That duplicate MCP path is redundant with the per-tool-call metadata
path and makes the sandbox-state contract larger than needed. The
existing managed network proxy refresh on sandbox-policy changes is
still needed, so this keeps that behavior separate from the removed MCP
notification.

## What Changed

- Removed the exported `MCP_SANDBOX_STATE_CAPABILITY` and
`MCP_SANDBOX_STATE_METHOD` constants.
- Removed detection of `codex/sandbox-state` during MCP initialization
and stopped sending `codex/sandbox-state/update` at server startup.
- Removed the `McpConnectionManager::notify_sandbox_state_change`
plumbing while preserving the managed network proxy refresh when a user
turn changes sandbox policy.
- Slimmed `McpConnectionManager::new` so startup paths pass only the
initial `SandboxPolicy` needed for MCP elicitation state.
- Kept `codex/sandbox-state-meta` support intact; servers that opt in
still receive the current `SandboxState` on tool-call request `_meta`
([remaining call
path](https://github.com/openai/codex/blob/ff2d3c1e72ff08ce13743b99605d19d338edd51c/codex-rs/core/src/mcp_tool_call.rs#L487-L526)).
- Added regression coverage for refreshing the live managed network
proxy on a per-turn sandbox-policy change.

## Verification

- `cargo test -p codex-core
new_turn_refreshes_managed_network_proxy_for_sandbox_change`
- `cargo test -p codex-mcp`

codex-rs/core/src/connectors.rs

@@ -1,8 +1,6 @@
 use std::collections::BTreeSet;
 use std::collections::HashMap;
 use std::collections::HashSet;
-use std::env;
-use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::LazyLock;
 use std::sync::Mutex as StdMutex;
@@ -42,7 +40,6 @@ use codex_login::default_client::is_first_party_chat_originator;
 use codex_login::default_client::originator;
 use codex_mcp::CODEX_APPS_MCP_SERVER_NAME;
 use codex_mcp::McpConnectionManager;
-use codex_mcp::SandboxState;
 use codex_mcp::ToolInfo;
 use codex_mcp::ToolPluginProvenance;
 use codex_mcp::codex_apps_tools_cache_key;
@@ -228,13 +225,6 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_options_and_status(
     let (tx_event, rx_event) = unbounded();
     drop(rx_event);
 
-    let sandbox_state = SandboxState {
-        sandbox_policy: SandboxPolicy::new_read_only_policy(),
-        codex_linux_sandbox_exe: config.codex_linux_sandbox_exe.clone(),
-        sandbox_cwd: env::current_dir().unwrap_or_else(|_| PathBuf::from("/")),
-        use_legacy_landlock: config.features.use_legacy_landlock(),
-    };
-
     let (mcp_connection_manager, cancel_token) = McpConnectionManager::new(
         &mcp_servers,
         config.mcp_oauth_credentials_store_mode,
@@ -242,7 +232,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_options_and_status(
         &config.permissions.approval_policy,
         INITIAL_SUBMIT_ID.to_owned(),
         tx_event,
-        sandbox_state,
+        SandboxPolicy::new_read_only_policy(),
         config.codex_home.to_path_buf(),
         codex_apps_tools_cache_key(auth.as_ref()),
         ToolPluginProvenance::default(),