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/tests/suite/search_tool.rs

@@ -703,6 +703,7 @@ async fn tool_search_indexes_only_enabled_non_app_mcp_tools() -> Result<()> {
                     disabled_reason: None,
                     startup_timeout_sec: Some(Duration::from_secs(10)),
                     tool_timeout_sec: None,
+                    default_tools_approval_mode: None,
                     enabled_tools: Some(vec!["echo".to_string(), "image".to_string()]),
                     disabled_tools: Some(vec!["image".to_string()]),
                     scopes: None,