tests: isolate approval fixtures from host rules (#18288)

## Why

Several approval-focused tests were unintentionally sensitive to
host-level rule files. On machines with broader allowed command
prefixes, commonly allowed commands such as `/bin/date` could bypass the
approval path these tests were meant to exercise, making the fixtures
depend on the developer or CI host configuration.

## What changed

- Pins the approval matrix fixture to the explicit user reviewer so it
does not inherit a host reviewer.
- Changes OTel approval fixtures to request `/usr/bin/touch
codex-otel-approval-test`, avoiding a command that may be pre-approved
by local rules.
- Clears the config layer stack for the permissions-message assertion
that needs to compare only the permissions text under test.

## Verification

- `env -u CODEX_SANDBOX_NETWORK_DISABLED cargo test -p codex-core --test
all approval_matrix_covers_all_modes -- --nocapture`
- `env -u CODEX_SANDBOX_NETWORK_DISABLED cargo test -p codex-core --test
all permissions_messages -- --nocapture`

codex-rs/core/tests/suite/approvals.rs

@@ -593,7 +593,7 @@ async fn submit_turn(
             final_output_json_schema: None,
             cwd: test.cwd.path().to_path_buf(),
             approval_policy,
-            approvals_reviewer: None,
+            approvals_reviewer: Some(ApprovalsReviewer::User),
             sandbox_policy,
             permission_profile: None,
             model: session_model,