FIX: mark truncated LiteLLM responses and handle empty truncation

[Chen Yufeiyang (feiiiiii5)]

Description

Fixes #2878. LiteLLM responses stopped by max_tokens were saved as complete answers; an empty truncated response raised EmptyResponseException and aborted the turn.

This follows the existing OpenAI target behavior using shared helpers: warn on finish_reason="length", mark the returned piece truncated, and return an empty piece with response_error="empty" when no content remains. Partial content, usage, finish reason, and response cost are retained. Non-truncated empty responses still raise.

Tests and Documentation

• pytest tests/unit/prompt_target/target/test_litellm_chat_target.py -q -rs: 84 passed, no skips. All 112 installed packages match uv.lock, including LiteLLM 1.102.1. This includes six local loopback wire tests through the real adapter; no external provider was used.
• Both new truncation regressions fail with the source restored to current main (2596f34): the partial piece is unmarked and the empty case raises. Removing the warning separately makes its assertion fail.
• Ruff check/format and scoped ty passed on both changed files. Full optional-dependency tests and live providers were not run.
• No documentation change: this applies the established truncation contract to another target.