fix(prompt): clarify tool-calling instructions to prevent fabricated Observation in lightweight LLMs

[phananh1010]

Summary

Lightweight LLMs sometimes generate Observation text without invoking a tool. This PR clarifies the system prompt to distinguish tool-use vs. final-answer cases, reducing hallucinated tool results.

Problem

The old prompt mixed Observation into the output template without clear separation. Smaller models often filled it in prematurely, fabricating results even when no tool was called (see #3154).

Solution

Update en.json prompt to:

• Define two explicit modes:
  1. Tool call → Thought, Action, Action Input, then blank Observation until tool returns.
  2. Final answer → Thought, Final Answer only.
• Add rules: no Final Answer when calling a tool; no non-empty Observation unless it is the actual tool output.

This removes ambiguity and enforces correct sequencing.