feat(database): add aggregate capability

[KilianBoute]

Description

Adds aggregation pipeline support as an optional database capability, enabling consumers to execute MongoDB aggregate queries through the shared database abstraction without coupling application code directly to the MongoDB driver.

This change improves the flexibility of the database layer by introducing a dedicated Aggregator interface that cleanly extends existing capabilities while keeping pipeline construction and decoding under consumer control. Both the real MongoDB client and the mock database now support aggregation, ensuring feature parity between production and test environments.

The mock implementation includes:

• aggregation call tracking
• queued and expected aggregate responses
• option propagation
• reset support for test isolation

Comprehensive tests were added to validate aggregate option filtering, result handling, error propagation, and mock behavior. This makes aggregation-based features easier to test reliably while preserving the lightweight and composable design of the current database package.

[codecov]

Codecov Report

❌ Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/database/mock.go	85.18%	4 Missing ⚠️
pkg/database/mongodb.go	60.00%	4 Missing ⚠️

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