feat(tools): add AgentPay x402 payment tools for ADK agents

Adds AgentPay tools that enable ADK agents to make autonomous HTTP
payments using the x402 protocol (HTTP 402 Payment Required).

Tools added:
- fetch_paid_api: Fetch a URL, auto-paying any x402 402 challenge
- get_wallet_info: Return wallet address, USDC balance, spend limits
- check_spend_limit: Pre-check payment against on-chain spend limits
- send_payment: Send USDC directly within autonomous spend policy

Implementation wraps the agentpay-mcp MCP server via stdio subprocess
(standard MCP transport). No custodian; non-custodial smart contract
wallet on Base. Spend limits enforced on-chain before any signing.

Standalone package:
  npm: https://www.npmjs.com/package/agentpay-mcp (patent pending)
  sdk: https://www.npmjs.com/package/agentwallet-sdk

See contributing/tools/agentpay/README.md for setup and usage.

Author: up2itnow0822

contributing/tools/agentpay/README.md

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+# AgentPay x402 Payment Tools for Google ADK
+
+Enable Google ADK agents to make autonomous HTTP payments using the
+[x402 payment protocol](https://www.x402.org/) — HTTP 402 Payment Required,
+handled automatically.
+
+Powered by [agentpay-mcp](https://www.npmjs.com/package/agentpay-mcp) and
+[agentwallet-sdk](https://www.npmjs.com/package/agentwallet-sdk) (patent pending).
+
+## What is x402?
+
+x402 is an open protocol that revives HTTP's original `402 Payment Required`
+status code. When an agent hits a paid API endpoint, the server returns a 402
+with payment details. The agent's wallet pays automatically and retries the
+request — no manual intervention, no API keys for every service.
+
+## What is AgentPay?
+
+AgentPay is a non-custodial smart contract wallet system for AI agents. Wallet
+ownership is represented by an NFT — your agent controls funds without a
+custodian. On-chain **spend limits** cap per-transaction and daily totals,
+protecting against runaway agent behavior.
+
+- **Chain:** Base (live), multi-chain coming
+- **Protocol:** x402 / HTTP 402
+- **NPM:** [`agentpay-mcp`](https://www.npmjs.com/package/agentpay-mcp) v4.0.0
+- **SDK:** [`agentwallet-sdk`](https://www.npmjs.com/package/agentwallet-sdk) v6.0.4
+- **Status:** Patent pending
+
+## Installation
+
+```bash
+# Python ADK package
+pip install google-adk-community
+
+# AgentPay MCP server (requires Node.js ≥ 18)
+npm install -g agentpay-mcp
+```
+
+## Setup
+
+```bash
+# Your AgentPay wallet private key (non-custodial)
+export AGENTPAY_PRIVATE_KEY="0x..."
+
+# Optional: custom RPC endpoint (defaults to Base mainnet)
+export AGENTPAY_RPC_URL="https://mainnet.base.org"
+```
+
+Get a wallet at [agentpay.xyz](https://agentpay.xyz) or via the
+`agentwallet-sdk` CLI.
+
+## Usage
+
+```python
+from google.adk.agents import Agent
+from google.adk_community.tools.agentpay import (
+    fetch_paid_api,
+    get_wallet_info,
+    check_spend_limit,
+    send_payment,
+)
+
+agent = Agent(
+    model="gemini-2.0-flash",
+    name="payment_agent",
+    description="An agent that can pay for API access autonomously using x402",
+    tools=[fetch_paid_api, get_wallet_info, check_spend_limit, send_payment],
+)
+
+# The agent can now call paid APIs, check its wallet, and send payments
+response = agent.run(
+    "Fetch the latest market data from https://api.example.com/market"
+    " and check my remaining daily spend limit."
+)
+```
+
+## Available Tools
+
+| Tool | Description |
+|------|-------------|
+| `fetch_paid_api` | Make an HTTP request; auto-pay x402 challenge if needed and retry |
+| `get_wallet_info` | Return wallet address, USDC balance, spend limits, and remaining allowance |
+| `check_spend_limit` | Pre-check whether a payment amount is within on-chain spend limits |
+| `send_payment` | Send USDC directly to an address within autonomous spend policy |
+
+## How `fetch_paid_api` Works
+
+```
+Agent calls fetch_paid_api(url="https://paid-api.example.com/data")
+  │
+  ├─ agentpay-mcp sends GET /data
+  │    Server returns HTTP 402 + payment details
+  │
+  ├─ agentpay-mcp pays on-chain (Base, USDC)
+  │    On-chain spend limit checked before signing
+  │
+  └─ agentpay-mcp retries GET /data with payment proof
+       Server returns 200 + data → returned to agent
+```
+
+## Security Model
+
+- **Non-custodial** — private key stays in your environment; no third party holds funds
+- **On-chain spend limits** — per-transaction and daily caps enforced by smart contract
+- **NFT ownership** — wallet controlled by NFT; rotate ownership without moving funds
+- **Audit trail** — every payment recorded on-chain
+
+## Links
+
+- [AgentPay NPM](https://www.npmjs.com/package/agentpay-mcp)
+- [agentwallet-sdk NPM](https://www.npmjs.com/package/agentwallet-sdk)
+- [x402 Protocol](https://www.x402.org/)
+- [ADK Integrations](https://google.github.io/adk-docs/integrations/)

src/google/adk_community/tools/__init__.py

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.

src/google/adk_community/tools/agentpay/__init__.py

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""AgentPay x402 payment tools for Google ADK.
+
+Enables ADK agents to make autonomous HTTP payments using the x402 protocol
+(HTTP 402 Payment Required) via the agentpay-mcp MCP server.
+
+See: https://www.npmjs.com/package/agentpay-mcp
+"""
+
+from .agentpay_tools import check_spend_limit
+from .agentpay_tools import fetch_paid_api
+from .agentpay_tools import get_wallet_info
+from .agentpay_tools import send_payment
+
+__all__ = [
+    "fetch_paid_api",
+    "get_wallet_info",
+    "check_spend_limit",
+    "send_payment",
+]