caplin: serialize uint64 beacon API fields as JSON strings

[BitWonka]

Summary

• Fixes caplin: execution_requests uint64 fields returned as JSON numbers, should be strings #20562.

• Per the beacon-APIs spec, Uint64 (and Gwei, which $refs Uint64) fields must be serialized as JSON strings. Several Caplin response types were emitting them as JSON numbers, causing spec-compliant clients to fail parsing.

  Behavior change: This is a spec-compliance fix. Consumers that were parsing these fields as JSON numbers will need to update, but those scripts were already non-portable, since every other CL client emits strings per spec

[Copilot]

Pull request overview

This PR fixes Caplin beacon API JSON serialization to comply with the beacon-APIs spec requirement that Uint64/Gwei fields be encoded as JSON strings (not numbers), addressing parsing failures in spec-compliant clients.

Changes:

• Update multiple Caplin response structs to serialize relevant uint64 fields as JSON strings via json:",string" tags.
• Add regression tests validating JSON output for DepositRequest, WithdrawalRequest, Pending* queue types, and validatorIdentityResponse.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cl/cltypes/solid/withdrawal.go	Encode WithdrawalRequest.Amount and PendingPartialWithdrawal uint64 fields as JSON strings.
cl/cltypes/solid/deposits.go	Encode DepositRequest/PendingDeposit uint64 fields as JSON strings.
cl/cltypes/solid/consolidation.go	Encode PendingConsolidation indices as JSON strings.
cl/beacon/handler/validators.go	Encode validatorIdentityResponse uint64 fields as JSON strings.
cl/cltypes/solid/withdrawal_test.go	Regression tests for JSON string serialization of withdrawal-related types.
cl/cltypes/solid/deposits_test.go	Regression tests for JSON string serialization of deposit-related types.
cl/cltypes/solid/consolidation_test.go	Regression test for JSON string serialization of PendingConsolidation.
cl/beacon/handler/validators_test.go	Regression test for JSON string serialization of validatorIdentityResponse.

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