Build LangChain Toolkit: Complete NEAR Integration

Description

## Overview
Create a comprehensive LangChain toolkit that enables LangChain agents to interact with NEAR Protocol - accounts, transactions, DeFi, and NFTs.

## Why This Matters
LangChain is the most popular agent framework. Native NEAR tools mean every LangChain agent can easily integrate NEAR functionality.

## Technical Requirements

### Toolkit Structure
```python
from langchain_near import NEARToolkit

# Initialize with account credentials
toolkit = NEARToolkit(
account_id="myagent.near",
private_key="ed25519:...",
network="mainnet" # or "testnet"
)

# Get all tools for an agent
tools = toolkit.get_tools()

# Or get specific tool categories
wallet_tools = toolkit.get_wallet_tools()
defi_tools = toolkit.get_defi_tools()
nft_tools = toolkit.get_nft_tools()
```

### Required Tools (minimum 15)

**Wallet Tools:**
- NEARBalanceTool - Check account balance
- NEARTransferTool - Send NEAR
- NEARAccountInfoTool - Get account details

**DeFi Tools:**
- RefSwapTool - Execute swaps on Ref Finance
- BurrowLendTool - Lend/borrow on Burrow
- MetaPoolStakeTool - Stake with Meta Pool
- YieldCompareTool - Compare DeFi yields

**NFT Tools:**
- NFTMintTool - Mint NFTs (Mintbase/Paras)
- NFTTransferTool - Transfer NFTs
- NFTMetadataTool - Get NFT info

**Contract Tools:**
- ContractViewTool - Call view methods
- ContractCallTool - Execute contract calls
- DeployContractTool - Deploy contracts

**Explorer Tools:**
- TransactionStatusTool - Check tx status
- AccountHistoryTool - Get tx history

### Code Quality
- Full type hints (Python 3.9+)
- Async support for all tools
- Comprehensive docstrings
- Unit tests with mocked NEAR responses
- Integration tests against testnet

## Deliverables
1. **PyPI package**: `langchain-near`
2. **15+ tools** across categories
3. **Example agents** using the toolkit
4. **Documentation** with usage patterns
5. **Listed on LangChain Hub**

## Success Criteria
- Installable via pip
- Works with LangChain 0.1+
- All tools function correctly
- 50+ downloads in first month

## Budget: 12 NEAR