add: collector transaction log retention and cleanup periodical

[amaan14999]

Description

Resolves #26267
Add a configurable retention threshold for collector fleet transaction log markers, with a daily periodical that purges expired entries. This prevents unbounded growth of the fleet_transaction_log collection.

Backend changes:

• Add collectorTransactionLogRetentionThreshold field to CollectorsConfig (default: 30 days)
• Add compound (created_at, _id) index in FleetTransactionLogService and new deleteOldMarkers() method
• Add getMinLastProcessedTxnSeq() aggregation to CollectorInstanceService to find the safe purge boundary
• Create PurgeCollectorFleetTransactionLogPeriodical (daily, leader-only) that deletes markers older than the retention threshold and below the minimum active sequence number
• Add retention field to CollectorsConfigRequest and validation in CollectorsConfigResource (must be positive, must be >= collector expiration period)
• Add telemetry entry in CollectorMetricsSupplier

Frontend changes:

• Add collector_transaction_log_retention_threshold to config types
• Add TimeUnitInput field in CollectorsSettings form with default of 30 DAYS

Tests:

• FleetTransactionLogServiceTest: 3 tests for deleteOldMarkers
• CollectorInstanceServiceTest: 4 tests for getMinLastProcessedTxnSeq
• PurgeCollectorFleetTransactionLogPeriodicalTest: 3 mock-based tests
• CollectorsConfigResourceTest: 3 validation tests (zero retention rejected, retention below expiration rejected, retention equal to expiration accepted)
• CollectorsConfigTest: 2 serialization/default tests
• CollectorsSettings.test.tsx: updated mock data and expected request with retention field

Motivation and Context

The fleet_transaction_log MongoDB collection grows unbounded as collector instances report status. Without cleanup, this collection accumulates stale markers indefinitely, consuming disk space and degrading query performance over time. This change introduces a configurable retention window so old markers are automatically purged.

Fixes #26267

How Has This Been Tested?

Unit tests (passing):

• PurgeCollectorFleetTransactionLogPeriodicalTest: 3/3 passed
• CollectorsConfigTest: 5/5 passed (including 2 new retention tests)
• CollectorsConfigResourceTest: 21/21 passed (including 3 new validation tests)
• CollectorsSettings.test.tsx: 7/7 passed

Integration tests (require Docker/TestContainers, not run locally):

• FleetTransactionLogServiceTest: 3 deleteOldMarkers tests
• CollectorInstanceServiceTest: 4 getMinLastProcessedTxnSeq tests

Type checking: tsc --noEmit passes with zero errors.
Compilation: ./mvnw compile passes with no errors from changed files.

Screenshots (if appropriate):

N/A - UI uses existing TimeUnitInput component, placed after the collector expiration field in Collectors Settings.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[CLAassistant]

All committers have signed the CLA.