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>

Author: guunergooner