Keep executor plugin MCP paths URI-native (#29628)

## Why

Executor-owned plugin roots are `PathUri`, but MCP config normalization
still converts them into a native `Path` using the app-server host's
rules. Relative `cwd` values can therefore resolve against the wrong
filesystem when host and executor path conventions differ.

This PR keeps executor MCP paths URI-native until the selected
environment launches the server, while retaining the existing host
parser behavior.

## What changed

- Keep one shared MCP normalization path with narrow host-`Path` and
executor-`PathUri` entrypoints.
- Preserve native host resolution for locally installed plugin MCP
configs.
- For executor configs, default `cwd` to the plugin root and resolve
relative working directories with the root URI's path convention.
- Accept explicit executor `file:` URIs only when they remain within the
selected plugin root.
- Preserve the selected environment id and existing remote
environment-variable ownership rules.
- Route the executor plugin provider through the URI-native entrypoint
without converting the root on the host.
- Ensure `codex doctor` does not probe executor-owned stdio commands or
foreign working directories on the host.
- Cover foreign Windows roots, relative and absolute executor working
directories, traversal rejection, runtime resolution, and doctor
behavior.

```text
plugin root:    file:///C:/plugins/demo
configured cwd: scripts
                  |
                  v
resolved cwd:  file:///C:/plugins/demo/scripts
                  |
                  v
launch through the selected executor
```

No new provider or filesystem abstraction is introduced.

## Stack

1. #29614 — add lexical `PathUri` containment.
2. #29620 — share URI-native manifest path resolution.
3. #28918 — keep selected plugin roots and resources URI-native.
4. #29626 — load executor skills without host path conversion.
5. **This PR** — resolve executor MCP working directories without host
path conversion.

codex-rs/ext/mcp/src/executor_plugin/provider.rs

@@ -2,8 +2,7 @@ use codex_config::McpServerConfig;
 use codex_config::McpServerTransportConfig;
 use codex_core_plugins::ResolvedExecutorPlugin;
 use codex_exec_server::ExecutorFileSystem;
-use codex_mcp::PluginMcpServerPlacement;
-use codex_mcp::parse_plugin_mcp_config;
+use codex_mcp::parse_executor_plugin_mcp_config;
 use codex_plugin::PluginResourceLocator;
 use codex_plugin::ResolvedPlugin;
 use codex_plugin::ResolvedPluginLocation;
@@ -11,7 +10,6 @@ use codex_plugin::manifest::PluginManifestMcpServers;
 use codex_utils_path_uri::PathUri;
 use codex_utils_path_uri::PathUriParseError;
 use std::io;
-use std::path::PathBuf;
 use thiserror::Error;
 
 const DEFAULT_MCP_CONFIG_FILE: &str = ".mcp.json";
@@ -116,17 +114,13 @@ async fn load_from_file_system(
             (contents, config_path)
         }
     };
-    let plugin_root_path = PathBuf::from(plugin_root.inferred_native_path_string());
-    let parsed = parse_plugin_mcp_config(
-        plugin_root_path.as_path(),
-        &contents,
-        PluginMcpServerPlacement::Environment { environment_id },
-    )
-    .map_err(|source| ExecutorPluginMcpProviderError::ParseConfig {
-        plugin_id: plugin_id.to_string(),
-        path: config_path,
-        source,
-    })?;
+    let parsed = parse_executor_plugin_mcp_config(plugin_root, &contents, environment_id).map_err(
+        |source| ExecutorPluginMcpProviderError::ParseConfig {
+            plugin_id: plugin_id.to_string(),
+            path: config_path,
+            source,
+        },
+    )?;
 
     for error in parsed.errors {
         tracing::warn!(