fix: submit customer credit redemption payload from POS

[my-dev-jour]

Summary

Fix customer-credit POS sales so the frontend submits the redemption payload the backend already expects.

Before this change:

• the payment dialog created Customer Credit entries with is_customer_credit and credit_details
• POSSale.vue stripped that metadata before submit
• submit_invoice() never received customer_credit_dict / redeemed_customer_credit
• pure customer-credit sales could not redeem credit correctly

This patch:

• preserves customer-credit metadata through the POS submit flow
• builds customer_credit_dict and redeemed_customer_credit in useInvoice.js
• excludes synthetic Customer Credit rows from normal invoice payments
• allows pure customer-credit POS sales to pass core POS payment-row validation only when POSNext has explicitly marked the invoice for redemption

Validation

• reproduced the bug on staging (frappe 16.12.0, erpnext 16.11.0, pos_next 1.15.0)
• applied the fix on staging and reran the same bench-console sale flow
• confirmed the invoice submitted successfully
• confirmed customer-credit redemption created a linked Journal Entry
• confirmed the saved invoice finished with outstanding_amount = 0

Issue

Closes #210