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](ff2d3c1e72/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/codex-mcp/src/lib.rs

@@ -36,9 +36,7 @@ pub use mcp::tool_plugin_provenance;
 pub use mcp::with_codex_apps_mcp;
 pub use mcp_connection_manager::CodexAppsToolsCacheKey;
 pub use mcp_connection_manager::DEFAULT_STARTUP_TIMEOUT;
-pub use mcp_connection_manager::MCP_SANDBOX_STATE_CAPABILITY;
 pub use mcp_connection_manager::MCP_SANDBOX_STATE_META_CAPABILITY;
-pub use mcp_connection_manager::MCP_SANDBOX_STATE_METHOD;
 pub use mcp_connection_manager::McpConnectionManager;
 pub use mcp_connection_manager::SandboxState;
 pub use mcp_connection_manager::ToolInfo;