fix: StreamingPromptHook panic when chat_history is empty

[iskng]

Summary

• Fixes panic when using .with_hook() on streaming prompts without prior chat history
• Fixes incorrect hook semantics where wrong message was passed as prompt

Problem

The streaming implementation's on_completion_call hook was incorrectly trying to get the prompt from chat_history.last() instead of using the current_prompt variable. This caused:

1. Panic when using .with_hook() without prior chat history (empty vec)
2. Wrong semantics - hook received last history message (could be an assistant message) instead of the actual user prompt being sent

Root Cause

The streaming architecture keeps current_prompt separate from chat_history until after the completion (line 267). But the hook code was trying to extract the prompt from chat_history.last(), which doesn't contain the current prompt at that point.

Fix

Use current_prompt directly in the hook call, and pass the full chat_history (which correctly represents prior conversation only):

// Before (buggy)
let prompt = reader.last().cloned().expect("...");
let chat_history_except_last = reader[..reader.len() - 1].to_vec();
hook.on_completion_call(&prompt, &chat_history_except_last, ...)

// After (fixed)
hook.on_completion_call(&current_prompt, &reader.to_vec(), ...)

Test plan

• cargo check -p rig-core passes
• cargo test -p rig-core --lib passes (172 tests)
• Manual test with streaming hook and empty history (no longer panics)

Fixes #1131

[joshua-mo-143]

Hi, thanks for the PR! I'll be testing this over today/tomorrow or so, whenever I get the chance.

[joshua-mo-143]

lgtm, tested locally