Fix noisy shutdown warnings from worker pool and bridge

[sueszli]

Two bugs caused spurious warnings on every clean shutdown:

1. RemoteWorkerPool::launch() cloned Arc<PythonWorkerPool> into the dispatcher task, keeping the refcount above 1. When shutdown() called Arc::try_unwrap(inner.pool) it always failed, skipping pool shutdown entirely (and with it, bridge shutdown). Fixed by holding Weak in the dispatcher and upgrading per-request instead.

2. WorkerBridgeServer reader tasks warn on stream errors, but killing worker processes necessarily closes their gRPC connections with a broken pipe. Fixed by adding a shutting_down flag to WorkerBridgeState, set via mark_shutting_down() before the worker-kill loop, so expected connection errors during planned shutdown are silenced. Unexpected errors during normal operation still warn.

Additionally mutes build warning:

   Compiling waymark v0.1.0 (/Users/sueszli/dev/waymark)
warning: unused import: `std::sync::OnceLock`
 --> src/observability.rs:5:5
  |
5 | use std::sync::OnceLock;
  |     ^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

[sueszli]

Reproduce with: https://gist.github.com/sueszli/70d1fc670e9e805eb1f6d42cc923d70d