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/app-server-protocol/schema/typescript/LegacyAppPathString.ts

@@ -11,10 +11,10 @@
  *
  * 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