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/chats.ts

@@ -886,8 +886,8 @@ export const makeChatsSocket = (config: SocketConfig) => {
 			const { onMutation } = newAppStateChunkHandler(false)
 			const { mutationMap } = await decodePatches(
 				name,
-				[{ ...encodeResult?.patch, version: { version: encodeResult?.state?.version } }],
-				initial ?? { version: 0, hash: Buffer.alloc(0), indexValueMap: {} },
+				[{ ...encodeResult!.patch, version: { version: encodeResult!.state.version } }],
+				initial!,
 				getAppStateSyncKey,
 				config.options,
 				undefined,