feat(mpp-seller): add Stripe charge offers

[mnebliienko]

Summary

• add a seller-side Stripe charge method backed by authenticated server configuration
• enforce exact USD cents and the supported amount range before challenge issuance
• normalize authoritative offer fields before policy evaluation and challenge generation
• preserve the native mppx validate/broadcast lifecycle and server-owned settlement

Verification

• pnpm typecheck
• pnpm lint
• pnpm test
• pnpm --filter @inflowpayai/mpp-seller build
• pnpm typedoc
• changed-line coverage: 59/59 executable lines
• authenticated localhost configuration round trips for configured and unavailable sellers

Stripe subscriptions are intentionally not advertised; this change supports one-time charge offers only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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