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/tui/src/history_cell/mcp.rs

@@ -414,7 +414,7 @@ pub(crate) fn new_mcp_tools_output(
                 lines.push(vec!["    • Command: ".into(), cmd_display.into()].into());
 
                 if let Some(cwd) = cwd.as_ref() {
-                    lines.push(vec!["    • Cwd: ".into(), cwd.display().to_string().into()].into());
+                    lines.push(vec!["    • Cwd: ".into(), cwd.to_string().into()].into());
                 }
 
                 let env_display = format_env_display(env.as_ref(), env_vars);