chore: alias builder id serdes helpers

[lodekeeper]

Addresses review feedback on #9593 by adding explicit builder ID serdes aliases while preserving existing validator helper names for validator call sites.

Changes:

• Add toBuilderIdsStr / fromBuilderIdsStr aliases in API serdes
• Use the builder aliases for getStateBuilders request serialization/parsing

Verification:

• pnpm lint
• pnpm build
• pnpm check-types

[gemini-code-assist]

Code Review

This pull request introduces toBuilderIdsStr and fromBuilderIdsStr as aliases for toValidatorIdsStr and fromValidatorIdsStr in packages/api/src/utils/serdes.ts. It then updates the state route definitions in packages/api/src/beacon/routes/beacon/state.ts to use these new builder-specific serialization and deserialization functions instead of the validator-specific ones. There are no review comments, so I have no feedback to provide.

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