path-uri: clarify host-native path conversion (#29501)

## Why

Downstream refactors are producing confusing code with this
functionality having a very generic name. Encoding the specific
conversion approach in the method name makes it clearer.

## What

Rename `PathUri::from_path` to `PathUri::from_host_native_path` and
update its Rust call sites.

codex-rs/exec-server/src/server/handler/tests.rs

@@ -27,7 +27,8 @@ fn exec_params_with_argv(process_id: &str, argv: Vec<String>) -> ExecParams {
     ExecParams {
         process_id: ProcessId::from(process_id),
         argv,
-        cwd: PathUri::from_path(std::env::current_dir().expect("cwd")).expect("cwd URI"),
+        cwd: PathUri::from_host_native_path(std::env::current_dir().expect("cwd"))
+            .expect("cwd URI"),
         env_policy: None,
         env: inherited_path_env(),
         tty: false,