feat(keychain): pass CustomerSession client secret to Stripe Elements

[broody]

Summary

• Add customerSessionClientSecret to GraphQL queries for both Stripe payment mutations
• Wire the new field through the credit purchase context to the Stripe Elements provider
• Update generated types to include the optional field

Depends on cartridge-gg/internal#4234

Test plan

• Verify Payment Element renders saved payment methods for returning customers (live mode)
• Verify test mode payments still work without customer session
• Verify "save payment method" checkbox appears for new payment methods

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
controller-example-next	Ready	Preview	Mar 31, 2026 11:13pm
keychain	Ready	Preview	Mar 31, 2026 11:13pm
keychain-storybook	Ready	Preview	Mar 31, 2026 11:13pm

[claude]

Claude finished @broody's task —— View job

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Documentation Analysis Complete

• Review PR changes to understand user-facing impact
• Check docs repository structure
• Determine if documentation updates are needed
• Update documentation if required

Analysis Result

No documentation updates needed.

This PR implements an internal enhancement to the Stripe payment flow that does not affect user-facing behavior or APIs:

What changed:

• Added customerSessionClientSecret field to GraphQL queries and TypeScript types
• Wired the field through internal context to Stripe Elements provider
• The change enables Stripe's saved payment methods feature for returning customers

Why no docs update is needed:

1. No API changes: No changes to controller.openStarterPack(), controller.openPurchaseCredits(), or any public SDK methods
2. Transparent to developers: Developers integrating Controller don't need to know about or configure this internal Stripe feature
3. Internal implementation detail: This is an internal keychain enhancement that improves UX automatically without requiring developer action
4. No configuration required: The feature works automatically when Stripe provides the customer session secret - no setup or code changes needed from integrators

The existing documentation in docs-repo/src/pages/controller/starter-packs.md already covers Stripe credit card payments at a high level, which remains accurate. This internal enhancement simply improves the payment experience for returning customers behind the scenes.

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