feat: admin audit events for set_paused, set_admin, set_token_allowed

[shogun444]

Summary

Closes #458

This PR adds dedicated audit events for admin actions so off-chain indexers and auditors can reconstruct the administrative history of the contract.

Changes

• Added paused_set event for set_paused
• Added admin_changed event for set_admin
• Added token_allowed_set event for set_token_allowed
• Emitted the new events immediately after successful state updates
• Preserved the existing two-topic event schema used throughout the contract
• Included the acting admin and event timestamp in the payload following the existing event conventions
• Updated the event documentation table in events.rs
• Replaced the existing symbol_short!("admin_action") usage with Symbol::new(env, ...) to match the current Soroban SDK requirements

Tests

• Added set_paused_emits_event
• Added set_admin_emits_event
• Added set_token_allowed_emits_event
• Added set_paused_failure_emits_no_event
• Verified emitted topics and payload contents match the expected schema

Notes

• No state transition logic was modified.
• Existing event ordering and topic layout remain unchanged for compatibility with current indexers.

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