chore: unit tests for runtime registry and RuntimeInfo error handling

[Goku2099]

This PR adds a couple of focused unit tests in the runtime core package.

1. Added unit tests for 'NewRuntimeRegistry' to verify runtime registration and dependency wiring.
2. Added a small unit test for 'TrainingRuntime.RuntimeInfo' to cover the unsupported 'runtimeTemplateSpec' error path.

Why this change

While looking at test coverage in 'pkg/runtime/core', I noticed that:

• 'registry.go' did not have direct unit test coverage.
• The error-handling branch in 'RuntimeInfo' was not exercised by existing tests, which mostly cover happy paths.

These tests aim to cover those gaps without touching production code or adding integration complexity.

Scope

• Test-only changes
• No behavior changes
• No production code modifications

related: #3152

Verification

go test ./pkg/runtime/core/...

[google-oss-prow]

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Details

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[coveralls]

Pull Request Test Coverage Report for Build 21643569736

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 51.754%

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Totals
Change from base Build 21610377640:	0.5%
Covered Lines:	1254
Relevant Lines:	2423

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💛 - Coveralls