test: guard relayer event id allocations

[ming1523]

Summary

This PR adds regression tests for relayer event ID allocation in relayer/src/core/event.rs.

Why

The file already contains a TODO to add coverage for event ID conflicts.

These tests make two invariants explicit:

• non-delegated event categories should not accidentally reuse each other's event IDs
• delegated user decrypt events should remain intentionally aligned with the user decrypt event ID space

This helps protect the routing contract encoded in event_id() without changing runtime behavior.

Changes

• add a test that checks the non-delegated event ID spaces are disjoint
• add a test that checks delegated user decrypt events stay mapped to the corresponding user decrypt IDs

Validation

• verified the change is limited to test code in relayer/src/core/event.rs
• cargo test was not run in this environment because the current machine does not have a usable Rust toolchain (cargo not found in PATH)

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[ming1523]

I have already signed the individual CLA on April 1, 2026. This PR is part of the same small cleanup series as #2226, so I am waiting for the CLA bot/config to be refreshed on this branch as well.

[github-actions]

This PR is unverified and has been open for 2 days, it will now be closed. If you want to contribute please sign the CLA as indicated by the bot.

[aquint-zama]

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