fix: migrate langchain shim imports to langchain_core

[functionxX]

Fixes #226

What changed

After the langchain 1.x restructure removed its re-export shims, from langchain.prompts.chat import ... / from langchain.schema... import ... break with ModuleNotFoundError: No module named 'langchain.prompts'. This migrates the affected imports to langchain_core, which is already a declared dependency (pyproject.toml) and already used elsewhere in the codebase (arkitect/core/component/llm/base.py, arkitect/core/component/output_parser/rag_output.py).

• arkitect/core/component/prompts/custom_prompt.py — langchain.prompts.chat → langchain_core.prompts.chat, langchain.schema.messages → langchain_core.messages
• arkitect/core/component/llm/llm.py — langchain.prompts.chat → langchain_core.prompts.chat, langchain.schema.output_parser → langchain_core.output_parsers
• arkitect/core/component/llm/utils.py — langchain.prompts.chat → langchain_core.prompts.chat
• demohouse/live_voice_call/backend/prompt.py — langchain.prompts.chat → langchain_core.prompts.chat

How verified

• py_compile on all four files
• Import smoke test after uv sync: all four modules import cleanly
• ruff check --select I clean on the touched files (the remaining E501 warnings are pre-existing on main)

[functionxX]

Hi — friendly ping on this. It fixes the langchain 1.x import crash (shim re-exports removed) across the 4 affected modules, verified with py_compile + import smoke + ruff. Would appreciate a review when you have a moment. Thanks!