test: add contract event emission tests for all state-changing functions

[devUnixx]

Closes #330

Changes

Added event emission tests to all three Soroban contracts, verifying that correct events are emitted with the right topics and data fields for every state-changing function.

energy_token

• test_mint_emits_event — verifies ("mint",) topic with (to, amount) data
• test_transfer_emits_event — verifies ("transfer",) topic with (from, to, amount) data
• test_burn_emits_event — verifies ("burn",) topic with (from, amount) data
• test_retire_emits_event — verifies ("retire",) topic is emitted

audit_registry

• test_anchor_emits_event — verifies ("anchor",) topic with (reading_hash, ledger_sequence, timestamp) data

community_governance

• test_propose_emits_event — verifies ("propose",) topic with proposal ID
• test_vote_emits_event — verifies ("vote",) topic with (proposal_id, voter, approve) data
• test_finalize_emits_event — verifies ("final",) topic with (proposal_id, status) data
• test_execute_emits_event — verifies ("exec",) topic with proposal ID

Acceptance criteria

• energy_token: mint, transfer, retire events tested
• audit_registry: anchor event tested
• community_governance: proposal_created, voted, executed events tested
• Event fields (topics, data) verified in tests

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