This is the second cleanup in the await-holding lint stack. The
higher-level goal, following #18178
and #18398, is to enable Clippy
coverage for guards held across `.await` points without carrying broad
suppressions.

The stack is working toward enabling Clippy's
[`await_holding_lock`](https://rust-lang.github.io/rust-clippy/master/index.html#await_holding_lock)
lint and the configurable
[`await_holding_invalid_type`](https://rust-lang.github.io/rust-clippy/master/index.html#await_holding_invalid_type)
lint for Tokio guard types.

Several existing fields used `tokio::sync::Mutex<()>` only as
one-at-a-time async gates. Those guards intentionally lived across
`.await` while an operation was serialized. A mutex over `()` suggests
protected data and trips the await-holding lint shape; a single-permit
`tokio::sync::Semaphore` expresses the intended serialization directly.

## What changed

- Replace `Mutex<()>` serialization gates with `Semaphore::new(1)` for
agent identity ensure, exec policy updates, guardian review session
reuse, plugin remote sync, managed network proxy refresh, auth token
refresh, and RMCP session recovery.
- Update call sites from `lock().await` / `try_lock()` to
`acquire().await` / `try_acquire()`.
- Map closed-semaphore errors into the existing local error types, even
though these semaphores are owned for the lifetime of their managers.
- Update session test builders for the new
`managed_network_proxy_refresh_lock` type.

## Verification

- The split stack was verified at the final lint-enabling head with
`just clippy`.





---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/18403).
* #18698
* #18423
* #18418
* __->__ #18403

## Why

This is part of the follow-up work from #18178 to make Codex ready for
Clippy's
[`await_holding_lock`](https://rust-lang.github.io/rust-clippy/master/index.html#await_holding_lock)
/
[`await_holding_invalid_type`](https://rust-lang.github.io/rust-clippy/master/index.html#await_holding_invalid_type)
lints.

This bottom PR keeps the scope intentionally small:
`NetworkProxyState::record_blocked()` only needs the state write lock
while it mutates the blocked-request ring buffer and counters. The debug
log payload and `BlockedRequestObserver` callback can be produced after
that lock is released.

## What changed

- Copies the blocked-request snapshot values needed for logging while
updating the state.
- Releases the `RwLockWriteGuard` before logging or notifying the
observer.

## Verification

- `cargo test -p codex-network-proxy`


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/18418).
* #18698
* #18423
* __->__ #18418