exec-server: require explicit filesystem sandbox cwd

[bolinfest]

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

---

Stack created with Sapling. Best reviewed with ReviewStack.

• tests: isolate approval fixtures from host rules #18288
• shell-escalation: carry resolved permission profiles #18287
• mcp: include permission profiles in sandbox state #18286
• tui: carry permission profiles on user turns #18285
• tui: sync session permission profiles #18284
• app-server: accept command permission profiles #18283
• protocol: report session permission profiles #18282
• rollout: persist turn permission profiles #18281
• clients: send permission profiles to app-server #18280
• -> exec-server: require explicit filesystem sandbox cwd #19046

[starr-openai]

cc @viyatb-oai since you were also working on this recently