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/code_mode.rs

@@ -239,6 +239,7 @@ async fn run_code_mode_turn_with_rmcp_config(
                 disabled_reason: None,
                 startup_timeout_sec: Some(Duration::from_secs(10)),
                 tool_timeout_sec: None,
+                default_tools_approval_mode: None,
                 enabled_tools: None,
                 disabled_tools: None,
                 scopes: None,