CBG-5020: fix panic in eviction based on out of sync stats

[gregns1]

CBG-5020

• Defensive checks when evicting based off of memory
• Moves some code around to have defer use for unlocking

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/173/

[Copilot]

Pull request overview

This PR addresses a panic in the revision cache eviction logic when memory statistics become out of sync with the actual cache state. The fix adds defensive checks when performing type assertions and introduces safeguards to detect and correct inconsistent memory statistics.

Key changes:

• Added defensive type assertion checks in cache removal operations
• Implemented detection and correction of inconsistent memory statistics during eviction
• Refactored locking to use defer statements for safer unlock behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
db/revision_cache_lru.go	Added type assertion checks, moved to defer-based locking, and added logic to detect/correct inconsistent memory stats during eviction
db/revision_cache_test.go	Added test case to reproduce and verify the fix for the panic scenario with out-of-sync stats

[bbrks]

It would be much clearer and potentially safer to use Set(0) for this than doing two separate Value and Add IMO

[gregns1]

I have changed this, I have added some comments on this area, we can;t quite set cacheMemoryBytesStat to 0 given this is the memory usage across all shards.