fix(openrouter): Extract usage from separate streaming chunk

[markritterman]

Problem

When using OpenRouter with streaming and stream_options.include_usage=true, usage data (token counts) is not captured in the StreamEndEvent.

Root Cause

OpenRouter sends usage data in a separate final chunk after the finish_reason chunk:

Chunk N-1: {"choices": [{"finish_reason": "stop", ...}]}           <- has finish_reason, NO usage
Chunk N:   {"choices": [{"finish_reason": null, ...}], "usage": {...}}  <- has usage, finish_reason is null

The current code only extracts usage when finish_reason !== null:

if ($rawFinishReason !== null) {
    // ...
    $usage = $this->extractUsage($data);  // Only runs when finish_reason is set
}

Since the usage chunk has finish_reason: null, usage extraction is skipped.

Solution

Move usage extraction outside the finish_reason check so it runs on every chunk. The extractUsage() method already safely returns null when there's no usage data.

Testing

Tested with OpenRouter API (via vLLM endpoint):

Before fix:

StreamEndEvent: usage = null

After fix:

StreamEndEvent: usage = {promptTokens: 27, completionTokens: 2, ...}

Impact

• No breaking changes
• Fixes streaming usage tracking for OpenRouter and any OpenAI-compatible API that sends usage in a separate chunk
• extractUsage() is idempotent and safe to call on every chunk