exec-server: require explicit filesystem sandbox cwd (#19046)

## Why

This is a cleanup PR for the `PermissionProfile` migration stack. #19016
fixed remote exec-server sandbox contexts so Docker-backed filesystem
requests use a request/container `cwd` instead of leaking the local test
runner `cwd`. That exposed the broader API problem:
`FileSystemSandboxContext::new(SandboxPolicy)` could still reconstruct
filesystem permissions by reading the exec-server process cwd with
`AbsolutePathBuf::current_dir()`.

That made `cwd`-dependent legacy entries, such as `:cwd`,
`:project_roots`, and relative deny globs, depend on ambient process
state instead of the request sandbox `cwd`. As later PRs make
`PermissionProfile` the primary permissions abstraction, sandbox
contexts should be explicit about whether they carry a request `cwd` or
are profile-only. Removing the implicit constructor prevents new call
sites from accidentally rebuilding permissions against the wrong `cwd`.

## What changed

- Removed `FileSystemSandboxContext::new(SandboxPolicy)`.
- Kept production callers on explicit constructors:
`from_legacy_sandbox_policy(..., cwd)`, `from_permission_profile(...)`,
and `from_permission_profile_with_cwd(...)`.
- Updated exec-server test helpers to construct `PermissionProfile`
values directly instead of routing through legacy `SandboxPolicy`
projections.
- Updated the environment regression test to use an explicit restricted
profile with no synthetic `cwd`.

## Verification

- `cargo test -p codex-exec-server`
- `just fix -p codex-exec-server`


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19046).
* #18288
* #18287
* #18286
* #18285
* #18284
* #18283
* #18282
* #18281
* #18280
* __->__ #19046

codex-rs/exec-server/src/environment.rs

@@ -443,8 +443,11 @@ mod tests {
             std::env::current_exe().expect("current exe").as_path(),
         )
         .expect("absolute current exe");
-        let sandbox = crate::FileSystemSandboxContext::new(
-            codex_protocol::protocol::SandboxPolicy::new_read_only_policy(),
+        let sandbox = crate::FileSystemSandboxContext::from_permission_profile(
+            codex_protocol::models::PermissionProfile::from_runtime_permissions(
+                &codex_protocol::permissions::FileSystemSandboxPolicy::restricted(Vec::new()),
+                codex_protocol::permissions::NetworkSandboxPolicy::Restricted,
+            ),
         );
 
         let err = environment