implemented the contract WASM upgrade function with version tracking.

[tecch-wiz]

I've implemented the contract WASM upgrade function with version tracking. Here's a summary of all changes made to fluxapay/src/lib.rs:

Changes Made

1. New DataKey::ContractVersion variant (line ~629)

Added a new storage key for persisting the contract version string.

2. New constant INITIAL_CONTRACT_VERSION (line ~633)

pub const INITIAL_CONTRACT_VERSION: &str = "1.0.0";

3. Updated initialize_payment_processor (line ~3677)

Now sets the initial contract version ("1.0.0") in persistent storage during initialization.

4. Updated version() and added get_version() in PaymentProcessor impl (line ~3651)

• version() now returns a String from persistent storage (falling back to "1.0.0")
• get_version() is an alias for version()

5. Updated upgrade_contract() in PaymentProcessor impl (line ~5762)

• Requires admin role authorization
• Captures the old version string
• Calls env.deployer().update_current_contract_wasm(new_wasm_hash) to replace the WASM
• Bumps the minor version (e.g., "1.0.0" → "1.0.1")
• Persists the new version string to DataKey::ContractVersion
• Emits CONTRACT/UPGRADED event with (old_version, new_version)

6. Added bump_version_string() helper function

Byte-level semver version bumping (no std string manipulation) that increments the patch number.

7. Added Error::UpgradeFailed = 48 variant

For host-level upgrade failures.

8. Added tests in fluxapay/src/test.rs

• test_upgrade_contract_version_and_event - verifies initial version and upgrade attempt
• test_upgrade_contract_unauthorized - verifies non-admin callers get rejected
• test_version_after_init - verifies version and get_version return "1.0.0" after init

Note: The pre-existing compilation errors (top_up_stream duplicate, MetadataTooLarge discriminant clash, merchant_registry_test.rs unmatched brace) are all pre-existing issues in the repository and not related to these changes.

closes #281

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