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/codex-mcp/src/mcp/mod.rs

@@ -276,6 +276,7 @@ fn codex_apps_mcp_server_config(config: &McpConfig, auth: Option<&CodexAuth>) ->
         disabled_reason: None,
         startup_timeout_sec: Some(Duration::from_secs(30)),
         tool_timeout_sec: None,
+        default_tools_approval_mode: None,
         enabled_tools: None,
         disabled_tools: None,
         scopes: None,