feat: progress export, multi-admin support, contract upgrade mechanism, and gas estimation (#211, #212, #213, #214)

[Kaycee276]

closes #211
closes #212
closes #213
closes #214

Summary of Changes

1. Progress Export Function (#211)

• Added/verified export_progress(learner, course_id) function in progress-tracker returning a complete ProgressExport struct containing enrollment status, enrollment timestamp, completed modules bitmap, total module count, quiz score results, quizzes submitted, total quiz score, overall progress, and credential eligibility in a single call without pagination.
• Added comprehensive unit test coverage in tests/unit/progress_tests.rs.

2. Multi-Admin Support (#212)

• Added AdminInfo struct (address, role) and Admins persistent storage key in learn-token/src/storage.rs.
• Implemented add_admin, remove_admin, and get_admins functions in learn-token allowing dynamic administration and role management (Admin, Minter, Pauser).
• Added multi-sig support for critical contract operations (execute_multisig_op, upgrade_multisig), requiring dual authorization from two distinct Admin role holders.
• Added integration test suite in tests/integration/admin_role_flow.rs.

3. Contract Upgrade Mechanism (#213)

• Implemented Soroban WASM contract upgrade mechanism with upgrade(new_wasm_hash) and upgrade_multisig(...) in contract logic.
• Managed version tracking (upgrade_version), recorded upgraded wasm_hash in state, and emitted upgraded contract event.
• Verified state preservation across upgrades in tests/integration/upgrade_tests.rs.

4. Gas Estimation Function (#214)

• Updated estimate_claim_gas(learner, course_id, quiz_id) in learn-token to return ClaimEstimate containing would_succeed, estimated_reward, failure_reason, and estimated_gas.
• Performed read-only simulation path without state mutations or authorization side-effects.

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