chat: make trailing newline optional after assistant tag in Laguna pa…

[ghost]

Overview

This PR relaxes the Laguna assistant generation prompt parsing template in common/chat.cpp to make the trailing newline after the <assistant> tag optional instead of strictly required.

Symptoms / Issues Resolved

Without this patch:

1. Parser Stalls: If a client tool or system template formats prompts slightly differently (e.g., omitting the trailing newline after <assistant> or adding spaces), the PEG parser fails to match the template.
2. Infinite Generation Hangs: When the parser fails to match the expected <assistant>\n literal, the server hangs indefinitely or fails to return the reasoning block.

This fix makes the Laguna parser robust against formatting differences, allowing successful prompt template matching whether a trailing newline, trailing spaces, or nothing follows the <assistant> tag.

Technical Changes

• Replaces the strict const std::string GEN_PROMPT = "<assistant>\n" string literal matching with a flexible regex-style pattern:
  auto gen_prompt = p.literal("<assistant>") + p.optional(p.space());
• Updates the reasoning and assistant output head matches (extract_reasoning) to use the new gen_prompt parser rule.

Requirements

• I have read and agree with the contributing guidelines.
• AI usage disclosure: YES - An AI assistant was used to locate the parsing expression boundaries and rewrite the PEG template rules in C++.