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/core-plugins/src/loader.rs

@@ -28,7 +28,6 @@ use codex_core_skills::config_rules::skill_config_rules_from_stack;
 use codex_core_skills::loader::SkillRoot;
 use codex_core_skills::loader::load_skills_from_roots;
 use codex_exec_server::LOCAL_FS;
-use codex_mcp::PluginMcpServerPlacement;
 use codex_mcp::parse_plugin_mcp_config;
 use codex_plugin::AppConnectorId;
 use codex_plugin::AppDeclaration;
@@ -1282,17 +1281,16 @@ async fn load_mcp_servers_from_file(
     let Ok(contents) = tokio::fs::read_to_string(mcp_config_path.as_path()).await else {
         return PluginMcpDiscovery::default();
     };
-    let parsed =
-        match parse_plugin_mcp_config(plugin_root, &contents, PluginMcpServerPlacement::Declared) {
-            Ok(parsed) => parsed,
-            Err(err) => {
-                warn!(
-                    path = %mcp_config_path.display(),
-                    "failed to parse plugin MCP config: {err}"
-                );
-                return PluginMcpDiscovery::default();
-            }
-        };
+    let parsed = match parse_plugin_mcp_config(plugin_root, &contents) {
+        Ok(parsed) => parsed,
+        Err(err) => {
+            warn!(
+                path = %mcp_config_path.display(),
+                "failed to parse plugin MCP config: {err}"
+            );
+            return PluginMcpDiscovery::default();
+        }
+    };
     for error in parsed.errors {
         warn!(
             plugin = %plugin_root.display(),
@@ -1311,11 +1309,7 @@ fn load_mcp_servers_from_manifest_object(
     plugin_root: &Path,
     object_config: &str,
 ) -> PluginMcpDiscovery {
-    let parsed = match parse_plugin_mcp_config(
-        plugin_root,
-        object_config,
-        PluginMcpServerPlacement::Declared,
-    ) {
+    let parsed = match parse_plugin_mcp_config(plugin_root, object_config) {
         Ok(parsed) => parsed,
         Err(err) => {
             warn!(