feat: add Stripe payment method (SPT charge intent)

[brendanjryan]

Summary

Adds Stripe payment method support to pympp, mirroring the mppx TypeScript implementation.

Key design decisions

• SPT flow: Uses Stripe's Shared Payment Token (private preview), matching mppx exactly
• Duck-typed client: Supports both client.v1.payment_intents (stripe-python v8+) and client.payment_intents (legacy) via _resolve_payment_intents()
• Two verification paths: Stripe SDK client or raw httpx POST to api.stripe.com
• Idempotency key: mppx_{challenge_id}_{spt} — matches TypeScript SDK for cross-SDK consistency
• Analytics metadata: Injects mpp_version, mpp_is_mpp, mpp_intent, etc. on every PaymentIntent
• transform_request hook: Injects networkId and paymentMethodTypes into challenge methodDetails
• Error classes: Uses VerificationFailedError (RFC 9457 Problem Details) instead of plain VerificationError
• Shared DEFAULT_TIMEOUT: Extracted to mpp._defaults — used by both Stripe and Tempo methods

Usage

# Server
from mpp.server import Mpp
from mpp.methods.stripe import stripe, ChargeIntent

server = Mpp.create(
    method=stripe(
        network_id="bn_...",
        payment_method_types=["card"],
        currency="usd",
        decimals=2,
        intents={"charge": ChargeIntent(secret_key="sk_...")},
    ),
)

# Client
from mpp.methods.stripe import stripe

method = stripe(
    create_token=my_spt_proxy,
    payment_method="pm_card_visa",
    intents={},
)

Files

File	Purpose
src/mpp/_defaults.py	Shared DEFAULT_TIMEOUT constant
src/mpp/methods/stripe/__init__.py	Package exports
src/mpp/methods/stripe/_defaults.py	Stripe API base URL
src/mpp/methods/stripe/client.py	Client-side credential creation (SPT flow)
src/mpp/methods/stripe/intents.py	Server-side verification (PaymentIntent)
src/mpp/methods/stripe/schemas.py	Pydantic schemas
tests/test_stripe.py	44 tests covering all paths
examples/stripe/	Working example (server + headless client)

mppx parity guards

• Rejects metadata.externalId in challenge (reserved for credential payload)
• Requires networkId in challenge.methodDetails
• Surfaces Stripe error details (message/code) in raw HTTP path exceptions

Test coverage (44 tests)

• Schema validation (payload, request, method details)
• Client credential creation (happy path, external ID, missing fields)
• Server verification (SDK client, modern v1 client, expiry, action required, failed status)
• Raw HTTP path (success, Stripe error parsing, non-JSON errors, metadata form encoding)
• Analytics metadata and idempotency key format
• Async context manager lifecycle (owned vs injected client)
• Factory defaults and parameter validation

Example (verified working)

examples/stripe/ contains a pay-per-fortune server + headless client using pm_card_visa. Tested end-to-end against Stripe test mode:

Client → GET /api/fortune → 402
Client → POST /api/create-spt → spt_... created
Client → GET /api/fortune + credential → pi_... succeeded → 🥠 fortune