sdks/ts: add MetadataJSON types to metadata pipeline

[Myestery]

Summary

• Add MethodMetadataJSON, ClassMetadataJSON, MetadataJSON type definitions to golem-ts-types-core
• Replace any with MetadataJSON in loadFromJson, removing all as any casts
• Use ClassMetadataJSON and MethodMetadataJSON in typegen saveAndClearInMemoryMetadata
• Use MetadataJSON in TypescriptTypeRegistry.register

Context

Split from #2845 — this PR adds proper types to the metadata JSON pipeline only (~40 lines changed across 3 files).

Test plan

• Existing SDK tests pass
• Typegen metadata generation produces the same output

[github-actions]

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[vigoo]

The failing integration tests show that this change breaks the sdk somehow, need to be fixed before merge