mcp: accept foreign absolute cwd for remote stdio (#29493)

## Why

Remote stdio MCP servers can run in an environment whose path convention
differs from the Codex host. A Windows cwd such as
`C:\Users\openai\share` is absolute for the executor but was rejected by
a POSIX orchestrator.

Built on #29501, now merged, which only clarifies the host-native
`PathUri` constructor name.

## What changed

- Deserialize MCP cwd values as `LegacyAppPathString` so config does not
apply host path rules.
- Interpret that spelling as host-native for local launches and convert
it to `PathUri` at executor launch.
- Skip host filesystem and command resolution checks for remote stdio in
`codex doctor`.
- Add host-independent config and executor-boundary coverage using the
foreign path convention for each test platform.

## Validation

- `just test -p codex-utils-path-uri -p codex-config -p codex-mcp -p
codex-rmcp-client` (408 passed)
- `just test -p codex-cli -p codex-rmcp-client` (372 passed)
- `cargo check --workspace --tests`
- `just test` (11,311 passed; 43 unrelated environment/timing failures)
- `just fix -p codex-cli -p codex-config -p codex-core -p codex-mcp -p
codex-mcp-extension -p codex-rmcp-client -p codex-tui`

codex-rs/core/src/config/config_tests.rs

@@ -89,6 +89,7 @@ use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::NetworkAccess;
 use codex_protocol::protocol::RealtimeVoice;
 use codex_protocol::protocol::SandboxPolicy;
+use codex_utils_path_uri::LegacyAppPathString;
 use serde::Deserialize;
 use tempfile::tempdir;
 
@@ -5554,6 +5555,7 @@ async fn load_global_mcp_servers_returns_empty_if_missing() -> anyhow::Result<()
 #[tokio::test]
 async fn replace_mcp_servers_round_trips_entries() -> anyhow::Result<()> {
     let codex_home = TempDir::new()?;
+    let expected_cwd = LegacyAppPathString::from_path(codex_home.path());
 
     let mut servers = BTreeMap::new();
     servers.insert(
@@ -5564,7 +5566,7 @@ async fn replace_mcp_servers_round_trips_entries() -> anyhow::Result<()> {
                 args: vec!["hello".to_string()],
                 env: None,
                 env_vars: Vec::new(),
-                cwd: Some(codex_home.path().to_path_buf()),
+                cwd: Some(expected_cwd.clone()),
             },
             environment_id: "remote".to_string(),
             enabled: true,
@@ -5603,7 +5605,7 @@ async fn replace_mcp_servers_round_trips_entries() -> anyhow::Result<()> {
             assert_eq!(args, &vec!["hello".to_string()]);
             assert!(env.is_none());
             assert!(env_vars.is_empty());
-            assert_eq!(cwd, &Some(codex_home.path().to_path_buf()));
+            assert_eq!(cwd, &Some(expected_cwd));
         }
         other => panic!("unexpected transport {other:?}"),
     }
@@ -6104,6 +6106,7 @@ async fn replace_mcp_servers_serializes_cwd() -> anyhow::Result<()> {
     let codex_home = TempDir::new()?;
 
     let cwd_path = PathBuf::from("/tmp/codex-mcp");
+    let cwd = LegacyAppPathString::from_path(&cwd_path);
     let servers = BTreeMap::from([(
         "docs".to_string(),
         McpServerConfig {
@@ -6112,7 +6115,7 @@ async fn replace_mcp_servers_serializes_cwd() -> anyhow::Result<()> {
                 args: Vec::new(),
                 env: None,
                 env_vars: Vec::new(),
-                cwd: Some(cwd_path.clone()),
+                cwd: Some(cwd.clone()),
             },
             environment_id: codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID.to_string(),
             enabled: true,
@@ -6147,7 +6150,7 @@ async fn replace_mcp_servers_serializes_cwd() -> anyhow::Result<()> {
     let docs = loaded.get("docs").expect("docs entry");
     match &docs.transport {
         McpServerTransportConfig::Stdio { cwd, .. } => {
-            assert_eq!(cwd.as_deref(), Some(Path::new("/tmp/codex-mcp")));
+            assert_eq!(cwd, &Some(LegacyAppPathString::from_path(&cwd_path)));
         }
         other => panic!("unexpected transport {other:?}"),
     }

codex-rs/core/src/config/edit/document_helpers.rs

@@ -69,7 +69,7 @@ fn serialize_mcp_server_table(config: &McpServerConfig) -> TomlTable {
                 entry["env_vars"] = array_from_env_vars(env_vars);
             }
             if let Some(cwd) = cwd {
-                entry["cwd"] = value(cwd.to_string_lossy().to_string());
+                entry["cwd"] = value(cwd.as_str());
             }
         }
         McpServerTransportConfig::StreamableHttp {