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/codex-mcp/src/plugin_config_tests.rs

@@ -7,6 +7,7 @@ use codex_config::McpServerConfig;
 use codex_config::McpServerEnvVar;
 use codex_config::McpServerOAuthConfig;
 use codex_config::McpServerTransportConfig;
+use codex_utils_path_uri::LegacyAppPathString;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;
 use std::collections::HashMap;
@@ -31,7 +32,7 @@ fn stdio_server(
             args: Vec::new(),
             env: None,
             env_vars,
-            cwd: Some(cwd.to_path_buf()),
+            cwd: Some(LegacyAppPathString::from_path(cwd)),
         },
         environment_id: environment_id.to_string(),
         enabled: true,