Allow cached catalogs to satisfy MCP startup readiness (#47935)

## Why

Required MCP servers can block startup while establishing a live connection even when a usable tool catalog is already cached.

## What changed

- Add per-server `startup_readiness = "catalog"` to expose cached tools while the connection starts, retaining `"connection"` as the default.
- Allow an eligible cached catalog to satisfy required-server startup validation and model binding capture. Explicit server or plugin requirements still wait for a live connection, and tool execution still requires the current connection.
- Check tool filters and model visibility when accepting cached catalogs, and preserve cached catalog revisions for binding reuse.
- Serialize the new setting, expose it in the configuration schema, and trace startup readiness at validation and model binding gates.

GitOrigin-RevId: a1cbdf1757ebb5824cffe2cc56408be49711c422

codex-rs/core-plugins/src/manager_tests.rs

@@ -981,6 +981,7 @@ async fn load_plugins_loads_default_skills_and_mcp_servers() {
                     environment_id: "local".to_string(),
                     enabled: true,
                     required: false,
+                    startup_readiness: Default::default(),
                     supports_parallel_tool_calls: false,
                     omit_tools_from: None,
                     disabled_reason: None,
@@ -1083,6 +1084,7 @@ enabled = true
                 environment_id: "local".to_string(),
                 enabled: true,
                 required: false,
+                startup_readiness: Default::default(),
                 supports_parallel_tool_calls: false,
                 omit_tools_from: None,
                 disabled_reason: None,
@@ -2186,6 +2188,7 @@ async fn load_plugins_uses_manifest_configured_component_paths() {
                     environment_id: "local".to_string(),
                     enabled: true,
                     required: false,
+                    startup_readiness: Default::default(),
                     supports_parallel_tool_calls: false,
                     omit_tools_from: None,
                     disabled_reason: None,
@@ -2525,6 +2528,7 @@ async fn load_plugins_ignores_manifest_component_paths_without_dot_slash() {
                 environment_id: "local".to_string(),
                 enabled: true,
                 required: false,
+                startup_readiness: Default::default(),
                 supports_parallel_tool_calls: false,
                 omit_tools_from: None,
                 disabled_reason: None,
@@ -2780,6 +2784,7 @@ fn capability_index_filters_inactive_and_zero_capability_plugins() {
         environment_id: "local".to_string(),
         enabled: true,
         required: false,
+        startup_readiness: Default::default(),
         supports_parallel_tool_calls: false,
         omit_tools_from: None,
         disabled_reason: None,