Adds EventLogField service wrapping main.eventlog.field.get and main.eventlog.field.list REST API v3 endpoints. The service follows the same pattern as ChatMessageField in the Task scope.

Usage example:

$serviceBuilder->getMainScope()->eventLogField()->list();
$serviceBuilder->getMainScope()->eventLogField()->get('timestampX');

Test plan

• make lint-cs-fixer — passed
• make lint-rector — passed
• make lint-phpstan — passed
• make lint-deptrac — passed
• make test-unit — passed (650 tests)
• make test-integration-main-eventlog — 4/9 pass (existing EventLog tests); 5 new EventLogField tests return HTTP 500 — main.eventlog.field.* not yet available on the test portal

Closes #394

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