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.rs

@@ -7,6 +7,7 @@ use serde::Deserialize;
 use serde::Serialize;
 use serde::Serializer;
 use std::fmt;
+use std::path::Path;
 use thiserror::Error;
 use ts_rs::TS;
 
@@ -18,10 +19,10 @@ use ts_rs::TS;
 ///
 /// When converting from [`PathUri`], "native" refers to the supplied
 /// [`PathConvention`], which may be foreign to the operating system running
-/// this process. The inner string is private so path-producing code must convert
-/// from [`AbsolutePathBuf`] or use [`Self::from_path_uri`] instead of bypassing
-/// the intended conversion boundary. Non-UTF-8 paths are converted to UTF-8
-/// lossily because this API value is serialized as a JSON string.
+/// this process. The inner string is private so path-producing code must use a
+/// path conversion method instead of bypassing the intended conversion
+/// boundary. Non-UTF-8 paths are converted to UTF-8 lossily because this API
+/// value is serialized as a JSON string.
 ///
 /// Deserialization accepts any UTF-8 string without interpreting or validating
 /// it. That unrestricted construction path is intentionally available only to
@@ -35,9 +36,14 @@ use ts_rs::TS;
 pub struct LegacyAppPathString(String);
 
 impl LegacyAppPathString {
+    /// Preserves path text without interpreting it using the current host.
+    pub fn from_path(path: &Path) -> Self {
+        Self(path.to_string_lossy().into_owned())
+    }
+
     /// Renders an absolute path using the current host's path convention.
     pub fn from_abs_path(path: &AbsolutePathBuf) -> Self {
-        Self(path.to_string_lossy().into_owned())
+        Self::from_path(path.as_path())
     }
 
     /// Renders a path URI using the requested native path convention.