Explicit regional placement disables WebSocket permessage-deflate while Smart Placement works

## Which Cloudflare product(s) does this pertain to?

Workers Runtime

## What versions & operating system are you using?

- Wrangler: 4.85.1
- OS: Linux

## Please provide a link to a minimal reproduction

I can provide a stripped repro if needed, but the key finding is that the only variable required to trigger this is Worker placement mode.

I reproduced it with:
- the stock Cloudflare Agents starter
- our real Worker app (`aidan-agent`)

## Describe the bug

WebSocket `permessage-deflate` negotiation works normally when a Worker has either:

- no `placement` configured, or
- Smart Placement enabled with:

```json
"placement": { "mode": "smart" }
```

But if I switch the Worker to an explicit regional placement pin:

```json
"placement": { "region": "aws:eu-west-2" }
```

then the WebSocket upgrade still succeeds with `101 Switching Protocols`, but `Sec-WebSocket-Extensions` disappears from the response.

So this is **not** a generic websocket compression failure. It appears to be specifically tied to **explicit regional placement**.

## Exact observed behaviour

### No placement configured
- `101 Switching Protocols`
- `Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits=15`

### Smart Placement
- `101 Switching Protocols`
- `Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits=15`

### Explicit regional placement (`aws:eu-west-2`)
- `101 Switching Protocols`
- `Sec-WebSocket-Extensions` header missing

## Steps to reproduce

1. Deploy a Worker that upgrades a request to a WebSocket.
2. Probe the same WebSocket endpoint with a request offering `permessage-deflate`.
3. Confirm that compression is negotiated when:
- `placement` is omitted, and
- `placement.mode = "smart"`
4. Change only the placement config to:

```json
"placement": { "region": "aws:eu-west-2" }
```

5. Redeploy.
6. Probe the same endpoint again.
7. Observe that the upgrade still returns `101`, but `Sec-WebSocket-Extensions` is no longer present.

## Expected behaviour

If the runtime supports websocket compression for this Worker, changing placement mode should not silently disable `permessage-deflate` negotiation.

## Actual behaviour

An explicit region pin appears to disable websocket compression negotiation, while Smart Placement and no placement do not.

## Extra context

- I originally discussed this with Sunil Pai on Discord, so cc @threepointone.
- There is already a broader issue about `permessage-deflate` not being accepted in some cases, but this report is narrower: the trigger here is specifically the **explicit placement pin**.
- As a workaround, we switched our production path to Smart Placement and compression immediately came back.