chore: document intentional await-holding cases (#18423)

## Why

This PR prepares the stack to enable Clippy await-holding lints that
were left disabled in #18178. The mechanical lock-scope cleanup is
handled separately; this PR is the documentation/configuration layer for
the remaining await-across-guard sites.

Without explicit annotations, reviewers and future maintainers cannot
tell whether an await-holding warning is a real concurrency smell or an
intentional serialization boundary.

## What changed

- Configures `clippy.toml` so `await_holding_invalid_type` also covers
`tokio::sync::{MutexGuard,RwLockReadGuard,RwLockWriteGuard}`.
- Adds targeted `#[expect(clippy::await_holding_invalid_type, reason =
...)]` annotations for intentional async guard lifetimes.
- Documents the main categories of intentional cases: active-turn state
transitions that must remain atomic, session-owned MCP manager accesses,
remote-control websocket serialization, JS REPL kernel/process
serialization, OAuth persistence, external bearer token refresh
serialization, and tests that intentionally serialize shared global or
session-owned state.
- For external bearer token refresh, documents the existing
serialization boundary: holding `cached_token` across the provider
command prevents concurrent cache misses from starting duplicate refresh
commands, and the current behavior is small enough that an explicit
expectation is easier to maintain than adding another synchronization
primitive.

## Verification

- `cargo clippy -p codex-login --all-targets`
- `cargo clippy -p codex-connectors --all-targets`
- `cargo clippy -p codex-core --all-targets`
- The follow-up PR #18698 enables `await_holding_invalid_type` and
`await_holding_lock` as workspace `deny` lints, so any undocumented
remaining offender will fail Clippy.

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

codex-rs/codex-api/src/endpoint/responses_websocket.rs

@@ -229,6 +229,10 @@ impl ResponsesWebsocketConnection {
 
         let current_span = Span::current();
         tokio::spawn(
+            #[expect(
+                clippy::await_holding_invalid_type,
+                reason = "the guard serializes exclusive use of the websocket stream for the lifetime of the response stream"
+            )]
             async move {
                 if let Some(model) = server_model {
                     let _ = tx_event.send(Ok(ResponseEvent::ServerModel(model))).await;