feat: add Stripe payment handler for Google Pay token processing

[paiking1]

Hey Google!

My name is Gal, I'm an ex-Googler myself, currently working at Sellersnap and we're very excited for the UCP launch, and we're currently in the waitlist.

Sellersnap is a company that provides repricing services for E-Commerce, so we're looking forward to allow our seller's items appear in Gemini. Let me know if it's possible for us to join the Beta asap, because we already have the core necessary logic implemented!

Below I'm sharing a PR to add Stripe support, because we've already implemented it in our codebase for UCP integration. Let me know what you think.

Thanks!

Summary

• Adds a Stripe PSP handler (payment_handlers/stripe_handler.py) that processes Google Pay credential tokens through Stripe PaymentIntent.create
• Routes google_pay handler in _process_payment to Stripe when STRIPE_SECRET_KEY is set, falls back to existing mock behavior otherwise
• Updates discovery_profile.json with Stripe tokenization gateway parameters
• Adds stripe>=7.0.0 as optional dependency
• Includes 15 unit + integration tests with full error handling coverage

This is the "last mile" that every merchant implementing UCP with a real PSP needs — the sample currently accepts Google Pay tokens without contacting a PSP. This PR shows the complete credential.token → stripe.PaymentIntent.create() flow.

Zero breaking changes: without STRIPE_SECRET_KEY, the server behaves exactly as before.

How to use

# Install stripe
uv sync --extra stripe

# Run with Stripe test mode
STRIPE_SECRET_KEY=sk_test_... uv run server.py \
   --products_db_path=/tmp/ucp_test/products.db \
   --transactions_db_path=/tmp/ucp_test/transactions.db \
   --port=8182

# Verify directly (creates real PaymentIntent in Stripe test dashboard)
STRIPE_SECRET_KEY=sk_test_... uv run python3 -c "
from payment_handlers.stripe_handler import StripePaymentHandler
handler = StripePaymentHandler()
print('PaymentIntent:', handler.process_token('tok_visa', 3500, 'USD'))
"

Tests

# Unit tests (no Stripe key needed) — 13 pass
uv run pytest payment_handlers/test_stripe_handler.py -v

# All tests including live Stripe API — 15 pass
STRIPE_SECRET_KEY=sk_test_... uv run pytest payment_handlers/test_stripe_handler.py -v

Test plan

• 13 unit tests pass without Stripe key (mocked)
• 2 live integration tests pass with sk_test_* key
• Real PaymentIntent created in Stripe test dashboard (pi_3T92T6KCyqI7L8TF0fm9FvWn)
• Server starts and /.well-known/ucp shows Stripe tokenization config
• simple_happy_path_client.py still works (steps 0–6, backward compatible)
• Without STRIPE_SECRET_KEY, google_pay tokens accepted as before (mock fallback)

🤖 Generated with Claude Code

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