Issue 9 — Test suite for contract event/log observation

### Scope
* Unit tests: EventParam factories, EventLog construction, ContractSubscription lifecycle, ContractListener provider resolution, event signature encoding, log decoding for all supported types.
* Integration tests against Hedera testnet in hiero-enterprise-test: deploy a small test contract emitting 1–2 event types, call functions that emit them, assert delivery and decoding through all three subscribeEvent overloads, and assert that closing the returned ContractSubscription stops further delivery.
* Failure-path tests: unsupported LedgerId throws as expected; double unsubscribe() doesn't throw; malformed logs don't crash the subscription.

### Acceptance Criteria
[x] ./mvnw verify passes end-to-end with test credentials configured.
[x] Coverage includes the plain API from both a direct base-module test and at least one test exercising it via an injected client in a Spring or Microprofile context.