feat(ai-chat) - Stop ai thinking if credits exhausted (#20526)

Billing is now decremented per-step, not per-turn. The onStepFinish
callback in chat-execution.service.ts calls a new
decrementAndCheckAvailableCredits method on each model step, so Redis is
debited incrementally as the agent runs rather than all at once at the
end.

Credit exhaustion stops the stream mid-run. When a step depletes the
remaining credits, a hasNoMoreAvailableCredits flag is set and passed
into the stopWhen predicate of streamText, causing the agent to halt
before starting the next step.

A new credits-exhausted event is introduced. After the stream drains and
the response is persisted, if credits ran out the job publishes a
dedicated credits-exhausted event to the frontend instead of the normal
message-persisted event.

The frontend handles this new event. useAgentChatSubscription has a new
credits-exhausted case that sets a BILLING_CREDITS_EXHAUSTED-coded error
on the atom, closes the writer, and stops the streaming state —
triggering the existing AiChatCreditsExhaustedMessage UI.

packages/twenty-server/src/engine/core-modules/logic-function/logic-function-executor/logic-function-executor.service.ts

@@ -376,7 +376,7 @@ export class LogicFunctionExecutorService {
 
       periodStart = currentPeriodStart;
 
-      await this.billingUsageService.decrementAvailableCredits({
+      await this.billingUsageService.decrementAvailableCreditsInCache({
         workspaceId,
         usedCredits: 100,
       });