test: add unit tests for 4 services (achievements-notifications, achievements, analytics, batch-size-guard)

[NteinPrecious]

Summary

This PR adds focused unit test suites for four previously untested services, satisfying the acceptance criteria of issues #1253–#1256.

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#1253 — achievements-notifications.service

Added src/achievements/achievements-notifications.service.spec.ts.

Covers:

• sendAchievementUnlockedNotification — marks notification sent; swallows repository errors gracefully
• sendBatchNotifications — returns correct count; returns 0 on empty result or repository failure
• resendFailedNotifications — retries up to 100 records; returns 0 on failure

Closes #1253

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#1254 — achievements.service

Added src/achievements/achievements.service.spec.ts (co-located with source, as required).

Covers:

• createAchievement — saves entity and invalidates cache
• getAchievementById — returns dto; throws NotFoundException when missing
• updateAchievement — updates and invalidates cache; throws NotFoundException when missing
• deactivateAchievement — sets isActive: false
• hasAchievement — returns true/false based on unlock record
• getUserAchievementCount — delegates to repository
• unlockAchievement — creates unlock event; skips duplicate; throws NotFoundException
• initializeProgress — reuses existing progress; creates when absent; throws NotFoundException
• getUserProgressForAchievement — returns dto; throws NotFoundException when absent

Closes #1254

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#1255 — analytics.service

Added src/analytics/analytics.service.spec.ts.

Covers:

• trackEvent — validates, batches, increments counter; re-throws BadRequestException; wraps unknown errors
• recordEvent and all assessment helpers — no-throw when counter is null
• getEvents — returns empty set; applies all optional filters
• getAnalyticsSummary — returns zeroed summary; maps raw query results into response shape

Closes #1255

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#1256 — batch-size-guard.service

Added src/analytics/services/batch-size-guard.service.spec.ts.

Covers:

• canAdd — returns true below max; returns false at and above max; increments dropped count only on rejection
• getDroppedCount — starts at zero; accumulates correctly
• resetDroppedCount — resets to zero; idempotent on zero

Closes #1256

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