Add support for Stripe Agentic Commerce Protocol (ACP) and Instant Checkout

## Overview
Stripe has launched the [Agentic Commerce Protocol (ACP)](https://stripe.com/newsroom/news/stripe-openai-instant-checkout), co-developed with OpenAI, which enables AI agents to facilitate seamless commerce transactions. This protocol powers Instant Checkout in ChatGPT and represents a significant shift toward agent-led transactions.

## Business Value
This feature would enable MemberJunction applications (particularly those used by associations and member-based organizations) to:
- Accept payments directly within AI-powered conversational interfaces
- Provide frictionless checkout experiences similar to ChatGPT's implementation
- Enable AI agents to act as "personal shoppers" for members
- Support modern commerce workflows without custom payment integrations

## Technical Details

### Agentic Commerce Protocol (ACP)
- **Open Standard**: Allows businesses to integrate once and work across multiple AI agents
- **Shared Payment Token (SPT)**: Applications can initiate payments without exposing buyer credentials
- **Scoped Security**: SPTs are scoped to specific merchants and cart totals
- **Merchant Control**: Merchants retain control over what's sold, branding, and order fulfillment

### Key Capabilities
1. **Inline Checkout**: Payment processing within conversational interfaces
2. **Multiple Payment Methods**: Supports various payment options including Stripe Link
3. **Tax & Fulfillment**: Built-in handling for tax calculation and order fulfillment
4. **Accept/Decline**: Merchant approval workflow for orders

## Proposed Implementation

### Phase 1: Core Actions
Create MemberJunction Actions to support ACP payment workflows:
- **Action: Initialize ACP Payment** - Create Shared Payment Token (SPT) for a transaction
- **Action: Process ACP Checkout** - Complete payment using SPT
- **Action: Verify ACP Transaction** - Validate and confirm payment status
- **Action: Handle ACP Webhook** - Process Stripe webhook events for order updates

### Phase 2: Angular Widget
Build a reusable Angular component for MJ applications:
- **Component: ``** - Single-button checkout widget
- Seamless integration with MJ's existing Angular ecosystem
- Configurable for different merchant configurations
- Support for MJ's security and user context

### Phase 3: AI Agent Integration
Extend MJ's AI agent capabilities to leverage ACP:
- Enable AI agents to initiate commerce transactions
- Support conversational shopping experiences
- Integration with existing MJ entities (Products, Orders, Payments, etc.)

## Use Cases for Associations
- **Event Registration**: AI-powered event recommendations with instant payment
- **Membership Renewals**: Conversational membership management with seamless payment
- **Merchandise Sales**: AI shopping assistant for association merchandise
- **Donation Processing**: Simplified donation workflows through AI interfaces

## Implementation Considerations
- Stripe API integration and authentication
- PCI compliance and security requirements
- Webhook handling for asynchronous events
- Testing with Stripe's sandbox environment
- Documentation and examples for developers

## References
- [Stripe ACP Announcement](https://stripe.com/newsroom/news/stripe-openai-instant-checkout)
- Stripe API Documentation (to be added when available)
- OpenAI ChatGPT integration examples

## Labels
- `enhancement`
- `payments`
- `ai-agents`
- `angular`
- `actions`

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**Note**: This represents Stripe's strategic vision of "building the economic infrastructure for AI" and positions MemberJunction to support next-generation commerce experiences.