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.

coderd/apidoc/swagger.json

@@ -11320,6 +11320,9 @@
 				},
 				"bridge": {
 					"$ref": "#/definitions/codersdk.AIBridgeConfig"
+				},
+				"chat": {
+					"$ref": "#/definitions/codersdk.ChatConfig"
 				}
 			}
 		},
@@ -12342,6 +12345,14 @@
 				}
 			}
 		},
+		"codersdk.ChatConfig": {
+			"type": "object",
+			"properties": {
+				"acquire_batch_size": {
+					"type": "integer"
+				}
+			}
+		},
 		"codersdk.ConnectionLatency": {
 			"type": "object",
 			"properties": {