Replace AGENT_CHAT_UNKNOWN_THREAD_ID with null for thread state (#19552)

## Summary
This PR refactors the AI chat thread state management to use `null`
instead of a sentinel string value (`AGENT_CHAT_UNKNOWN_THREAD_ID`) to
represent an uninitialized or new chat thread. This improves type safety
and makes the code more idiomatic by using `null` to represent the
absence of a value.

## Key Changes
- **Removed sentinel constant**: Deleted `AGENT_CHAT_UNKNOWN_THREAD_ID`
constant and replaced all usages with `null`
- **Updated state types**: Changed `currentAIChatThreadState`,
`agentChatLastDiffSyncedThreadState`, and
`agentChatDisplayedThreadState` to use `string | null` type with `null`
as default value
- **Updated component family states**: Modified message-related state
families to accept `threadId: string | null` instead of `threadId:
string`
- **Refined null checks**: Added explicit `null` checks in:
- `AgentChatMessagesFetchEffect`: Updated `isNewThread` logic to check
for `null` first
- `useAIChatThreadClick` and `useSwitchToNewAIChat`: Added guards to
only save drafts when `currentAIChatThread !== null`
- `AgentChatThreadInitializationEffect`: Added null check before UUID
validation
- `useEnsureAgentChatThreadIdForSend`: Added null check before comparing
with draft key
- **Updated fallback logic**: Used nullish coalescing operator (`??`) in
`AIChatTab` and `useAIChatEditor` to default to
`AGENT_CHAT_NEW_THREAD_DRAFT_KEY` when thread is null
- **Enhanced refetch safety**: Added early return in
`handleRefetchMessages` to prevent refetching when in new thread state

## Implementation Details
- The change maintains backward compatibility by treating `null` the
same way the code previously treated `AGENT_CHAT_UNKNOWN_THREAD_ID`
- All draft saving operations now safely check for null before
attempting to store drafts
- The nullish coalescing pattern (`currentAIChatThread ??
AGENT_CHAT_NEW_THREAD_DRAFT_KEY`) ensures proper fallback behavior when
accessing draft storage

https://claude.ai/code/session_01Pz8KCygSNgBPYsndbMq8f7

---------

Co-authored-by: Claude <noreply@anthropic.com>

packages/twenty-front/src/modules/ai/components/AgentChatThreadInitializationEffect.tsx

@@ -88,7 +88,10 @@ export const AgentChatThreadInitializationEffect = () => {
   }, [storeEntry.status, hasAiSettingsPermission, setAgentChatThreadsLoading]);
 
   useEffect(() => {
-    if (hasInitializedAgentChatThreads || isValidUuid(currentAIChatThread)) {
+    if (
+      hasInitializedAgentChatThreads ||
+      (currentAIChatThread !== null && isValidUuid(currentAIChatThread))
+    ) {
       return;
     }