feat(loader): enhance single active backend by treating as singleton

[mudler]

Description

This PR tries to increase stability when using single active backend option

Notes for Reviewers

Signed commits

• Yes, I signed my commits.

[Copilot]

Pull Request Overview

This PR enhances the handling of a single active backend by introducing a singleton mode in the ModelLoader. Key changes include adding lock/unlock methods to manage singleton access, updating various consumers to pass the singleton flag and use defer to release locks on errors, and adjusting the options initialization accordingly.

Reviewed Changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/model/initializers.go	Added Close and lockBackend methods to manage the singleton lock
core/http/routes/localai.go	Updated endpoints to use variable ml instead of a new loader instance
core/http/endpoints/openai/assistant_test.go	Updated NewModelLoader call to include the single backend flag
core/http/endpoints/localai/stores.go	Added defer sl.Close() to ensure lock release in endpoints
core/cli/*	Updated ModelLoader initializations to pass the single backend flag
core/backend/*	Added defer loader.Close() calls and updated options handling
core/application/*	Updated loader initialization and load conditions based on singleton mode

Comments suppressed due to low confidence (2)

core/http/routes/localai.go:53

• The variable 'ml' is used but not defined in this scope. Consider initializing 'ml' or renaming it back to the previously declared 'sl'.

router.Post("/stores/set", localai.StoresSetEndpoint(ml, appConfig))

core/backend/token_metrics.go:26

• The error message contains a spelling mistake ('loadmodel'); consider changing it to "could not load model" for clarity.

return nil, fmt.Errorf("could not loadmodel model")

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