refactor: separate test-only scalers from production code

[dshmatov]

Remove external-mock scaler from production buildScaler switch and implement a test scaler registry pattern instead.

Previously, the external-mock scaler (used only for testing) was defined in the production buildScaler() switch statement, mixing test and production code. This resolves the TODO comment at line 182.

Changes:

• Remove external-mock case and TODO from buildScaler()
• Add RegisterTestScalerBuilder() function for test-only scalers
• Register external-mock in BeforeSuite via the registry pattern
• Add unit tests to verify registry functionality

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Ensure make generate-scalers-schema has been run to update any outdated generated files.
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

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[JorTurFer]

Interesting approach! I'm thinking if we should register all the scaler builders as a map and just invoke them when needed. a map should be almost performant as a switch and it can remove all the testing code from the prod code registering the prod ones here and the extra one in the test file. WDYT @kedacore/keda-core-maintainers
@kedacore/keda-core-contributors

[dshmatov]

@JorTurFer yeah, good idea to replace this huge switch block! I can try to implement if everyone good with this approach

[dshmatov]

I've implemented the full registry pattern - replaced the entire switch statement with map-based registration. All production scalers now register via init(), test scalers use the same RegisterScalerBuilder() API, and buildScaler() is now just a simple map lookup.