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/cli/src/doctor.rs

@@ -3457,6 +3457,26 @@ mod tests {
         }));
     }
 
+    #[tokio::test]
+    async fn mcp_check_does_not_probe_environment_stdio_on_the_host() {
+        let remote_server: McpServerConfig = toml::from_str(
+            r#"
+                command = "remote-only-command"
+                environment_id = "remote"
+                cwd = "C:\\plugins\\demo"
+                required = true
+                env_vars = [{ name = "REMOTE_ONLY_TOKEN", source = "remote" }]
+            "#,
+        )
+        .expect("remote MCP config");
+        let servers = HashMap::from([("remote".to_string(), remote_server)]);
+
+        let check = mcp_check_from_servers(&servers).await;
+
+        assert_eq!(check.status, CheckStatus::Ok);
+        assert_eq!(check.summary, "MCP configuration is locally consistent");
+    }
+
     #[test]
     fn provider_specific_auth_allows_non_openai_provider_without_env_key() {
         let check = provider_specific_auth_check(