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/core/src/unified_exec/mod.rs

@@ -30,8 +30,8 @@ use std::sync::Weak;
 use codex_network_proxy::NetworkProxy;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_tools::UnifiedExecShellMode;
-use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_output_truncation::TruncationPolicy;
+use codex_utils_path_uri::PathUri;
 use rand::Rng;
 use rand::rng;
 use tokio::sync::Mutex;
@@ -96,8 +96,8 @@ pub(crate) struct ExecCommandRequest {
     pub process_id: i32,
     pub yield_time_ms: u64,
     pub max_output_tokens: Option<usize>,
-    pub cwd: AbsolutePathBuf,
-    pub sandbox_cwd: AbsolutePathBuf,
+    pub cwd: PathUri,
+    pub sandbox_cwd: PathUri,
     pub turn_environment: TurnEnvironment,
     pub shell_mode: UnifiedExecShellMode,
     pub network: Option<NetworkProxy>,
@@ -156,7 +156,7 @@ struct ProcessEntry {
     process: Arc<UnifiedExecProcess>,
     call_id: String,
     process_id: i32,
-    cwd: AbsolutePathBuf,
+    cwd: PathUri,
     initial_exec_command_active: Arc<std::sync::atomic::AtomicBool>,
     hook_command: String,
     tty: bool,