CBG-4981: fix panic in _config?include_runtime=true endpoint

[gregns1]

CBG-4981

• Have a nil check around sg replicate manager before attempting to fetch replications

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/167/

[Copilot]

Pull Request Overview

This PR fixes a panic in the _config?include_runtime=true endpoint (CBG-4981) that occurs when the SGReplicateMgr is nil. This can happen when a database fails to complete its online startup process, leaving certain components uninitialized.

Key changes:

• Added a nil check before accessing database.SGReplicateMgr in the config endpoint handler
• Added a regression test that simulates a failed database startup and verifies the endpoint doesn't panic

Reviewed Changes

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

File	Description
rest/admin_api.go	Added nil check for SGReplicateMgr before fetching replications to prevent panic
rest/api_test.go	Added regression test that creates a database with invalid config, triggers failed startup, and verifies the config endpoint works without panicking