fix(types): resolve all TypeScript compilation errors from non-null assertion removal

Fixes:
- chats.ts: revert encodeResult/initial to ! (guaranteed by callback assignment)
- groups.ts: fix operator precedence with ?? (wrap +attrs in parens), fix groupId type
- messages-recv.ts: extract messageKey.id to local const with fallback
- socket.ts: revert onClose! (guaranteed by synchronous callback assignment)
- event-buffer.ts: add 'notify' fallback for MessageUpsertType
- messages.ts: add filePath const after null guard, fix contextInfo type assertion
- noise-handler.ts: restore array index ! (guaranteed by length check)

Build now compiles with zero new errors.

https://claude.ai/code/session_01E2cfX1N3sJgCJBTvzGazSG

src/Socket/socket.ts

@@ -552,11 +552,7 @@ export const makeSocket = (config: SocketConfig) => {
 		})
 
 		if (sendMsg) {
-			if (!onClose) {
-				throw new Error('onClose handler not initialized')
-			}
-
-			sendRawMessage(sendMsg).catch(onClose)
+				sendRawMessage(sendMsg).catch(onClose!)
 		}
 
 		return result