fix(runtime-core): preserve snake case keys

[misaelalves99]

What changed

• Preserve object properties that are already in snake_case when converting keys with asSnakeCase.
• Keep camelCase-to-snake_case conversion behaviour unchanged.
• Promote the existing regression case from an expected failure to a normal passing test.

Fixes #1635

Regression proof

Before the implementation change, enabling the existing regression test produced:

• expected: { age: 30, first_name: "Jane", last_name: "Doe" }
• received: { last_name: "Doe" }

After the fix, the regression test passes normally.

Validation

• pnpm exec vitest run --project runtime-core — 5 test files passed, 35 tests passed.
• pnpm --filter @opendatacapture/runtime-core exec tsc --noEmit — passed.
• pnpm --filter @opendatacapture/runtime-core exec eslint src — passed.
• pnpm exec prettier --check packages/runtime-core/src/utils.ts packages/runtime-core/src/__tests__/utils.test.ts — passed.
• pnpm --filter @opendatacapture/runtime-core build — passed, including API Extractor.
• pnpm-lock.yaml remained unchanged.