Build LangChain Tool: NEAR Transaction Builder

Description

## Overview
Create a LangChain tool that helps agents construct and simulate NEAR transactions before execution.

## Why This Matters
Agents need to build complex transactions safely. This tool provides transaction construction with validation.

## Technical Requirements
- Build transfer transactions
- Construct function calls
- Batch multiple actions
- Simulate before sending
- Gas estimation

## Deliverables
1. PyPI package
2. Transaction templates
3. Simulation capability
4. Clear error messages

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