Centralize thread MCP connections in McpRuntime (#33889)

## What changed

- Add a thread-owned `McpRuntime` that atomically publishes connection-set
  replacements and owns shutdown.
- Route `McpResourceClient` snapshots and cache keys through that runtime so
  extension clients observe refreshed MCP servers.
- Keep model-step configuration snapshots separate from ownership of live MCP
  connections.

## Testing

- Add coverage that captures an extension's session resource client, refreshes
  the MCP server configuration, and verifies that the existing client sees the
  newly published server.

GitOrigin-RevId: 351b2e25594b8eb6b91e50ff8d06b7d49768ae1c

codex-rs/codex-mcp/src/lib.rs

@@ -10,6 +10,7 @@ pub use resource_client::McpResourceClientCacheKey;
 pub use resource_client::McpResourcePage;
 pub use resource_client::McpResourceReadResult;
 pub use rmcp_client::MCP_SANDBOX_STATE_META_CAPABILITY;
+pub use runtime::McpRuntime;
 pub use runtime::McpRuntimeContext;
 pub use runtime::SandboxState;
 pub use tool_catalog_cache::McpToolCatalogCache;