Validate generation batch limits - #1681

What changed

• Return an empty stream for max_tokens=0 and reject negative limits.
• Validate every per-prompt option list before creating caches or mutating batch state.

Root cause

Zero-token generation reached final response construction with uninitialized loop variables. Batch insertion used unchecked zip, silently truncating prompts when any option list was short.

Validation

• Focused zero-token and short-list regressions pass.

Fixes #1689