feat: configure acquire chat batch size (#23196)

## Summary
- add a hidden deployment config option for chat acquire batch size
(`CODER_CHAT_ACQUIRE_BATCH_SIZE` / `chat.acquireBatchSize`)
- thread the configured value into chatd startup while preserving the
existing default of `10`
- clamp the deployment value to the `int32` range before passing it into
chatd
- regenerate the API/docs/types/testdata artifacts for the new config
field

## Why
`chatd` currently acquires pending chats in batches of `10` via a
compile-time default. This change makes that batch size
operator-configurable from deployment config, so we can tune acquisition
behavior without another code change.

cli/testdata/server-config.yaml.golden

@@ -752,6 +752,11 @@ workspace_prebuilds:
   # limit; disabled when set to zero.
   # (default: 3, type: int)
   failure_hard_limit: 3
+# Configure the background chat processing daemon.
+chat:
+  # How many pending chats a worker should acquire per polling cycle.
+  # (default: 10, type: int)
+  acquireBatchSize: 10
 aibridge:
   # Whether to start an in-memory aibridged instance.
   # (default: false, type: bool)