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/config/src/mcp_edit_tests.rs

@@ -29,6 +29,7 @@ async fn replace_mcp_servers_serializes_per_tool_approval_overrides() -> anyhow:
             disabled_reason: None,
             startup_timeout_sec: None,
             tool_timeout_sec: None,
+            default_tools_approval_mode: Some(AppToolApproval::Auto),
             enabled_tools: None,
             disabled_tools: None,
             scopes: None,
@@ -62,6 +63,7 @@ async fn replace_mcp_servers_serializes_per_tool_approval_overrides() -> anyhow:
         r#"[mcp_servers.docs]
 command = "docs-server"
 supports_parallel_tool_calls = true
+default_tools_approval_mode = "auto"
 
 [mcp_servers.docs.tools]