feat: upgrade LangChain to v1

[chat-bandit]

Upgrade all LangChain packages from v0.3.x to v1.x (v2.x for google-genai).

⚙️ Under the Hood

• Upgraded all @langchain/* packages to latest major versions:
  • langchain 0.3 → 1.x
  • @langchain/core 0.3 → 1.x
  • @langchain/anthropic 0.3 → 1.x
  • @langchain/openai 0.6 → 1.x
  • @langchain/google-genai 0.2 → 2.x
  • @langchain/groq 0.2 → 1.x
  • @langchain/ollama 0.2 → 1.x
• Fixed DynamicStructuredTool import path from langchain/tools → @langchain/core/tools
• Added explicit return type annotations (TTools, DynamicStructuredTool) to resolve TS2742 errors caused by internal module re-exports in @langchain/core v1
• Removed obsolete patches/@langchain+ollama+0.2.4.patch (hardcoded index: 0 fix — resolved upstream in v1)

🧯 Risk and Rollback

• Risk Level: Medium — major version bump across 7 packages
• Known Risks: Streaming and tool-calling behavior may differ subtly at runtime despite passing typecheck
• Rollback Plan: Revert PR and npm install
• Mitigations: Lint + typecheck pass; core APIs (bindTools, stream, invoke, message types) are preserved in v1