unified-exec: preserve PathUri through exec-server (#28681)

## Why

It should be possible for app-server to handle "foreign" OS paths in
unified_exec working directories, allowing e.g. a Linux app-server to
run processes on e.g. a Windows exec-server.

## What

Convert the core unified_exec cwd values to use `PathUri`.

Adds fallible path conversion in several places to try to minimize the
scope of this change. The only time this change suppresses errors from
converting `PathUri` to an `AbsolutePathBuf` is when the turn is
configured with no sandboxing at all to allow us to make progress
testing without sandboxing.

Future changes to apply_patch and sandboxing will clean up these error
paths.

A tool's cwd is resolved from joining a model-provided workdir to the
environment's cwd. When using `AbsolutePathBuf::join()`, an
absolute-path workdir would overwrite the environment's cwd and we would
resolve permissions/sandboxing against the model-provided path. This
change extends `PathUri::join()` to also treat an absolute rhs as an
override of the base/lhs.

This also removes some coverage from the remove_env_windows tests until
a follow-up converts foreign paths in command exec events correctly.

## Breaking Changes

When using `AbsolutePathBuf::join()` for workdir resolution, we ended up
resolving tilde-prefixed paths against the app-server's `$HOME`, e.g.
`~/foo/bar` becomes `/home/anp/foo/bar`. It's difficult to do this with
`PathUri` joining, so after offline discussion this PR no longer
implements it.

A quick check of some power users' rollouts suggests that models don't
actually generate home-prefixed absolute working directories for their
spawns, so this shouldn't have any real blast radius.

codex-rs/exec/tests/suite/sandbox.rs

@@ -60,7 +60,12 @@ async fn spawn_command_under_sandbox(
         child.arg0(arg0);
     }
     child.args(args);
-    child.current_dir(exec_request.cwd);
+    // TODO(anp): Keep PathUri through the macOS sandbox process launch boundary.
+    let native_cwd = exec_request
+        .cwd
+        .to_abs_path()
+        .map_err(|err| io::Error::other(err.to_string()))?;
+    child.current_dir(native_cwd);
     child.env_clear();
     child.envs(exec_request.env);