feat: Integrate Stellar Asset Interface (SAI) Wrapper

[emmanuelist]

Summary

Implements the Stellar Asset Interface (SAI) Wrapper contract to allow the lab to accept XLM/USDC payments for premium certificates via the Soroban token interface.

Closes #131

Changes

New: contracts/src/sai_wrapper.rs

A standalone Soroban contract that wraps native Stellar Asset Contracts (SAC) to provide a payment gateway for premium certificates:

• init(admin, treasury, asset_contract, cert_price) — One-time setup with admin, treasury, initial accepted asset, and price
• payment_gateway(payer, asset_contract) — Core payment function that:
  1. Requires payer authorization
  2. Validates the asset is in the accepted registry
  3. Pre-flight checks payer balance (fails fast before transfer)
  4. Transfers cert_price tokens from payer → treasury via SAC token::Client
  5. Verifies transfer completion (treasury balance increased by exact amount)
  6. Records payment status and emits tracking event
• has_paid(payer) — Query whether a payer has completed payment
• get_balance(asset, account) — Wrapped balance lookup via SAC
• Multi-asset support — add_accepted_asset / remove_accepted_asset for admin-managed asset registry (XLM + USDC + any SAC)
• set_cert_price(admin, asset, price) — Admin-only price updates per asset
• get_total_collected(asset) — Total payments collected per asset

Bug fixes (pre-existing)

• token.rs: Fixed unclosed delimiter in burn() — duplicate authorization check left the if block unclosed
• token.rs: Fixed double require_auth in update_uri() — only_owner() already calls require_auth, second call triggered Auth::ExistingValue error
• tests.rs: Fixed unclosed delimiter in batch_issue_gas_efficiency() — duplicate for loop left a dangling open brace
• lib.rs: Fixed events().publish() call in batch_issue() — was passing 3 arguments instead of 2

Tests

14 new tests covering all SAI wrapper requirements:

Test	Scenario
payment_transfers_tokens_and_records_paid	Full payment flow: transfer + balance verification + has_paid
total_collected_tracks_payments	Cumulative payment tracking
has_paid_returns_false_for_unknown_address	Default state check
payment_rejects_insufficient_balance	Payer with 0 tokens rejected
payment_rejects_unaccepted_asset	Unregistered SAC rejected
multi_asset_payments	XLM + USDC (second SAC) both accepted
admin_can_update_cert_price	Price update flow
non_admin_cannot_update_price	Unauthorized price change rejected
admin_can_remove_asset	Asset deregistration
double_init_panics	Double initialization prevented
get_balance_returns_correct_amount	Wrapped balance query
get_treasury_returns_configured_address	Treasury query
init_rejects_zero_price	Zero price validation
set_cert_price_rejects_negative	Negative price validation

Full suite: 102 tests pass, 0 failures.

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[emmanuelist]

Note on CI failures: The 4 failing checks (Backend CI, Frontend CI, Backend Lint, Frontend Lint) are pre-existing upstream issues unrelated to this PR.

• This PR only modifies smart contract files under contracts/ — zero frontend or backend changes.
• The Smart Contracts (Clippy + rustfmt) check passes ✅.
• The same CI and Lint checks are failing on main itself (e.g. Lint Add comprehensive code formatting and project improvements #88, CI [Contract] Emergency Pause Switch #139 on commit 8b3414f; Lint fix: ai-powered-hackathon-idea-generator-service (closes #5) #86, CI [Contract] Implement Ownable Pattern for RsTokenContract #137 on commit c5df738) and on every other open PR (feature(contract): Ownable Patter nof RsTokenContract #183, Temporary storage for session verification #182, [Docs] Improve Project README with Detailed Setup Guide #185).
• Specific pre-existing errors:
  • Frontend ESLint: Unexpected any in soroban.ts:53 + unused vars in dashboard/page.tsx, simulator/page.tsx, certificates/[id]/page.tsx
  • Backend Prettier: Cannot find package 'prettier-plugin-tailwindcss' — missing dependency
  • Backend CI: TypeScript type errors in learning.service.ts and auth.ts

[emmanuelist]

@ayomideadeniran , kindly merge this PR. Thanks