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/utils/path-uri/src/api_path_string_tests.rs

@@ -496,6 +496,23 @@ fn foreign_absolute_syntax_deserializes_without_host_interpretation() {
     }
 }
 
+#[test]
+fn from_path_preserves_foreign_absolute_path_for_uri_conversion() {
+    #[cfg(not(windows))]
+    let (foreign_path, expected_uri) = (r"C:\Users\openai\share", "file:///C:/Users/openai/share");
+    #[cfg(windows)]
+    let (foreign_path, expected_uri) = ("/home/openai/share", "file:///home/openai/share");
+
+    let path: PathUri = LegacyAppPathString::from_path(std::path::Path::new(foreign_path))
+        .try_into()
+        .expect("foreign absolute path should convert");
+
+    assert_eq!(
+        path,
+        PathUri::parse(expected_uri).expect("valid expected URI")
+    );
+}
+
 #[test]
 fn renders_an_absolute_path_using_the_host_convention() {
     #[cfg(unix)]