Add Agent Plugins MCP config parsing (#36796)

## What changed

- Add `parse_agent_plugin_mcp_config` to translate Agent Plugins v1 `mcp.json` files into Codex MCP server configuration.
- Normalize `stdio` and streamable HTTP transports, including `PLUGIN_ROOT` and `PLUGIN_DATA` expansion, contained plugin paths, secure endpoint validation, and filtering of client-owned HTTP headers.
- Keep valid sibling servers when another server is invalid, while returning per-server parse errors.
- Preserve UTF-8 HTTP header values when forwarding streamable HTTP protocol headers.

## Testing

- Cover transport mapping, placeholder expansion, path containment, schema and field validation, per-server errors, platform-specific environment handling, and UTF-8 headers.

GitOrigin-RevId: ed4ab0fcf495afbb381ce48beb93989629444c56

codex-rs/codex-mcp/src/plugin_config.rs

@@ -10,6 +10,11 @@ use std::collections::BTreeMap;
 use std::path::Path;
 use tracing::warn;
 
+#[path = "agent_plugin_config.rs"]
+mod agent_plugin_config;
+
+pub use agent_plugin_config::parse_agent_plugin_mcp_config;
+
 #[derive(Clone, Copy, Debug)]
 enum PluginMcpSource<'a> {
     Host {