fix: duplicate litellm stream call doubles provider requests

[quevedoSteven]

Summary

The streaming path in fetch_response_litellm_openai called litellm.acompletion twice with the same kwargs. The first result was assigned to a variable that is never read — only the second call's stream was returned.

Impact

Every streamed request consumed two provider requests (e.g. two NVIDIA NIM requests) with the same API key:

• Halves the effective per-key rate limit (a 40 req/min NIM key only supports 20 streamed turns/min).
• With multi-key round-robin rotation, both calls share the same rotated key, so rotation spreads turns — not requests — and a busy session (several concurrent agents) quickly exhausts both keys → 429 → LLMRateLimited: Rate limit after 3 attempts.
• The first call's stream is abandoned, leaking an unconsumed stream on every request.

Change

Keep a single litellm.acompletion call for the streamed path, and the same fix in the tool_call_id truncation retry branch (which had the identical double-call pattern).

Verification

• stream=True: one acompletion call; the returned stream is the only request sent.
• stream=False: unchanged.
• Syntax checked (ast.parse); no references to the removed ret variable remain.

[quevedoSteven]

Superseded by #477 which includes the required NIM rotation code from #475.