core: stop threading SandboxPolicy through exec (#25700)

## Why

#25450 attempts a broad `SandboxPolicy` removal across several unrelated
surfaces, which makes it hard to review and still leaves new helper code
moving legacy policies around. This PR is a narrower alternative:
migrate only the exec-side Windows sandbox plumbing so the review can
focus on one production path and one compatibility boundary.

The goal is to stop threading `SandboxPolicy` through exec code without
expanding the migration into app-server, protocol, telemetry, config, or
session behavior.

## What changed

- Removed `ExecRequest::compatibility_sandbox_policy()`.
- Changed the Windows restricted-token and elevated filesystem override
helpers to accept `PermissionProfile` plus the split filesystem/network
policies instead of a `SandboxPolicy`.
- Kept the remaining legacy projection local to the writable-root
comparison that still needs to compare split policy behavior against the
legacy Windows backend model.
- Rejected restricted split filesystem policies that still grant
full-disk writes before using the Windows restricted-token backend,
preserving the previous clear-failure behavior for profiles that project
to `ExternalSandbox`.
- Updated the Windows sandbox override tests to exercise the new call
shape and cover the full-write split-profile regression.

## Verification

- `just test -p codex-core windows_restricted_token`
- `just test -p codex-core windows_elevated`

codex-rs/core/src/spawn.rs

@@ -30,7 +30,7 @@ pub enum StdioPolicy {
     Inherit,
 }
 
-/// Spawns the appropriate child process for the ExecParams and SandboxPolicy,
+/// Spawns the appropriate child process for the exec params and sandbox settings,
 /// ensuring the args and environment variables used to create the `Command`
 /// (and `Child`) honor the configuration.
 ///