fix(openai): fix stop_reason null, zero usage fields and max_tokens forwarding

- Fix stop_reason always null in assembled AssistantMessage by applying
  the value captured from message_delta event
- Reset partialMessage to null after message_stop to prevent duplicate
  AssistantMessage emission causing doubled content in next API request
- Forward computed maxTokens into buildOpenAIRequestBody as max_tokens
  so OpenAI-compatible endpoints receive the intended output cap
- Extract assembleFinalAssistantOutputs helper to deduplicate message
  assembly logic between message_stop handler and post-loop fallback
- Fix test helper to use events parameter instead of hidden global
- Add regression test for max_tokens request forwarding

Signed-off-by: guunergooner <tongchao0923@gmail.com>

src/utils/formatBriefTimestamp.ts

@@ -76,6 +76,7 @@ function getLocale(): string | undefined {
   }
 }
 
+/** Return the epoch-ms of the start of the local calendar day for `d`. */
 function startOfDay(d: Date): number {
   return new Date(d.getFullYear(), d.getMonth(), d.getDate()).getTime()
 }