Fix #1026: handle DELETE_OPERATION_TOO_LARGE in LogBatchPurger

[RajPabnani03]

Summary

Fixes #1026 — LogBatchPurger.execute() performed a single all-or-none delete on the batch scope, so when the DML cascaded to too many related records at once (DELETE_OPERATION_TOO_LARGE on LogEntry__c or FeedItem records), the entire batch failed with an unhandled exception and none of the scope was purged.

LogBatchPurger now wraps the delete in hardDeleteRecords(scopeRecords) and, on a System.DmlException, retries via hardDeleteRecordsWithPartialDeletes(scopeRecords):

List<Database.DeleteResult> deleteResults = LoggerDataStore.getDatabase().deleteRecords(scopeRecords, false);
// successful rows -> emptyRecycleBin(deletedRecords)
// failed rows      -> Logger.error(LogMessage, failedDeleteResults) when EnableLoggerSystemMessages is on

• Records that can be deleted are still purged within the same execute() call — a single record with an oversized cascade no longer blocks the rest of the batch.
• Records that genuinely can't be deleted produce handled Logger.error entries (gated on EnableLoggerSystemMessages, matching the class's existing convention) instead of "Failed to process batch" unhandled-exception emails; they're re-attempted on the next scheduled run.
• Adds a virtual emptyRecycleBin(List<SObject>) method to LoggerDataStore.Database (and routes hardDeleteRecords through it) so the whole delete chain stays mockable; hardDeleteRecord/hardDeleteRecords signatures are unchanged.

Adds it_should_partially_delete_records_when_batch_delete_throws_dml_exception to LogBatchPurger_Tests, using a new MockPartiallyFailingDatabase that throws on the all-or-none delete and returns mixed results on the partial delete.

Testing

Verified in a scratch org (Enterprise Edition, API v67): full nebula-logger/core deploy succeeded and the complete local Apex suite passed — 1260/1260 tests, 93% org-wide coverage — including the new it_should_partially_delete_records_when_batch_delete_throws_dml_exception (mocked LoggerDataStore.Database covering the fallback path).