CBG-2413: remove memory leak in cbgt.Cfg implementations

[torcolvin]

CBG-2413: remove memory leak in cbgt.Cfg implementations

If there are pending events from FireEvent but there are no consumers, the FireEvent will hang when <-cbgt.CbgEvent is called.

• Create a Stop method with a cancellable context so that any sends are cancelled
• create CfgSGMem which contains a cbgt.CfgMem implementation in Sync Gateway by using a shim that supports either DataStore or a map. The CfgSGMem is the CfgMem implementation.

Pre-review checklist

• 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

Integration Tests

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/1046/

[factory-droid]

Droid finished @torcolvin's task —— View job

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[Copilot]

Pull request overview

This PR addresses a goroutine leak/hang scenario in cbgt.Cfg event delivery by making Sync Gateway’s CfgSG event sending cancellable and by replacing the CE cbgt.NewCfgMem usage with an in-repo, stoppable in-memory CfgSG backend.

Changes:

• Introduces a cancellable context + Stop() on base.CfgSG to unblock pending FireEvent/Refresh sends.
• Adds an in-memory cfgMemoryStorage backend and exposes it via base.NewCbgtCfgMem(ctx) as a CE replacement for cbgt.NewCfgMem.
• Wires DatabaseContext and tests to use the new CfgSG-based cfg and stop it during shutdown.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
db/database.go	Switches CfgSG to *base.CfgSG, uses new NewCbgtCfgMem(ctx), and stops it on shutdown.
base/sg_cluster_cfg.go	Adds cancellable context + Stop(), introduces cfgMemoryStorage, and updates event delivery to be cancel-aware.
base/sg_cluster_cfg_test.go	Adds coverage for Get/Set/Del semantics, event delivery, and Stop() unblocking behavior across backends.
base/heartbeat_test.go	Updates tests to pass context into NewCbgtCfgMem(ctx).
base/dcp_test.go	Updates tests to pass context into NewCbgtCfgMem(ctx).
base/cbgt.go	Removes the old cbgt.NewCfgMem wrapper in favor of the new CfgSG-based implementation.

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[factory-droid]

One correctness issue to address: CfgSG.Stop currently does not stop cfgNodePoller because the poller is started with the non-cancellable ctx, so polling goroutines can survive database stop/offline. The other candidate items were test robustness suggestions rather than concrete defects.