core: derive active permission profiles (#18277)

## Why

`Permissions` should not store a separate `PermissionProfile` that can
drift from the constrained `SandboxPolicy` and network settings. The
active profile needs to be derived from the same constrained values that
already honor `requirements.toml`.

## What changed

This adds derivation of the active `PermissionProfile` from the
constrained runtime permission settings and exposes that derived value
through config snapshots and thread state. The app-server can then
report the active profile without introducing a second source of truth.

## Verification

- `cargo test -p codex-core --test all permissions_messages --
--nocapture`
- `cargo test -p codex-core --test all request_permissions --
--nocapture`



























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[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/18277).
* #18288
* #18287
* #18286
* #18285
* #18284
* #18283
* #18282
* #18281
* #18280
* #18279
* #18278
* __->__ #18277

codex-rs/app-server/src/codex_message_processor.rs

@@ -10207,6 +10207,11 @@ mod tests {
             approval_policy: codex_protocol::protocol::AskForApproval::OnRequest,
             approvals_reviewer: codex_protocol::config_types::ApprovalsReviewer::User,
             sandbox_policy: codex_protocol::protocol::SandboxPolicy::DangerFullAccess,
+            permission_profile:
+                codex_protocol::models::PermissionProfile::from_legacy_sandbox_policy(
+                    &codex_protocol::protocol::SandboxPolicy::DangerFullAccess,
+                    std::path::Path::new("/tmp"),
+                ),
             cwd: test_path_buf("/tmp").abs(),
             ephemeral: false,
             reasoning_effort: None,