fix(contracts): fix #392 event hub emit gas and #395 flash loan reentry tests

[Dopezapha]

Closes #395
Closes #392

PR Description

Fixes #392 by optimizing the Event Hub capture path so each captured event is stored by event ID instead of rewriting a growing persistent event log vector on every emit. This keeps the emit/archive write path constant time while preserving existing event query behavior for indexers.

Fixes #395 by adding focused Flash Loan reentrancy coverage with malicious receiver contracts that attempt nested single and batch flash loans during callbacks. The flash loan entry points now also use a temporary-storage reentrancy lock as defense in depth, while the tests assert Soroban rejects same-contract reentry.

Changed

• [Smart Contracts] Event Hub: Optimize gas usage on emit #392: Store Event Hub archive entries as DataKey::EventLogEntry(event_id) and keep get_events, get_event, and get_events_by_contract compatible.
• [Smart Contracts] Event Hub: Optimize gas usage on emit #392: Update the cross-contract event topic to xcontract to satisfy Soroban symbol_short! length limits.
• [Smart Contracts] Flash Loan: Add reentrancy guard tests #395: Add temporary-storage reentrancy lock around flash_loan and flash_loan_batch.
• [Smart Contracts] Flash Loan: Add reentrancy guard tests #395: Add malicious receiver tests for nested single-loan and batch-loan reentry attempts.

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