Add server-level approval defaults for custom MCP servers (#17843)

## Summary
- Add `default_tools_approval_mode` support for custom MCP server
configs, matching the existing `codex_apps` behavior
- Apply approval precedence as per-tool override, then server default,
then `auto`
- Update config serialization, CLI display, schema generation, docs, and
tests

## Testing
- `cargo check -p codex-config`
- `cargo check -p codex-core`
- `just write-config-schema`
- `just fmt`
- `cargo test -p codex-config`
- Targeted `codex-core` tests for config parsing, config writes, and MCP
approval precedence
- `just fix -p codex-config -p codex-core`

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

@@ -183,6 +183,7 @@ async fn load_plugins_loads_default_skills_and_mcp_servers() {
                     disabled_reason: None,
                     startup_timeout_sec: None,
                     tool_timeout_sec: None,
+                    default_tools_approval_mode: None,
                     enabled_tools: None,
                     disabled_tools: None,
                     scopes: None,
@@ -520,6 +521,7 @@ async fn load_plugins_uses_manifest_configured_component_paths() {
                 disabled_reason: None,
                 startup_timeout_sec: None,
                 tool_timeout_sec: None,
+                default_tools_approval_mode: None,
                 enabled_tools: None,
                 disabled_tools: None,
                 scopes: None,
@@ -630,6 +632,7 @@ async fn load_plugins_ignores_manifest_component_paths_without_dot_slash() {
                 disabled_reason: None,
                 startup_timeout_sec: None,
                 tool_timeout_sec: None,
+                default_tools_approval_mode: None,
                 enabled_tools: None,
                 disabled_tools: None,
                 scopes: None,
@@ -788,6 +791,7 @@ fn capability_index_filters_inactive_and_zero_capability_plugins() {
         disabled_reason: None,
         startup_timeout_sec: None,
         tool_timeout_sec: None,
+        default_tools_approval_mode: None,
         enabled_tools: None,
         disabled_tools: None,
         scopes: None,