bug: handle empty Fireworks AI stream choices

[mangopep]

Description: (required)

• Fixes Unhandled exception in fireworks-ai streaming requests when stream chunk has empty choices #1627.
• Handles Fireworks AI streaming chunks where choices is empty or missing.
• Preserves usage-only chunks without throwing.

Before Fix
The new reproduction test fails because the transformer reads parsedChunk.choices[0].index when choices is empty.

[![before-fix]]

After Fix
The same test passes after safely handling empty or missing choices.

![after-fix]

Tests Run/Test cases added: (required)

• Added test for usage-only Fireworks stream chunk with empty choices
• npx jest src/providers/fireworks-ai/chatComplete.test.ts --runInBand
• npm run build

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)