Added SSZ support for validator balance responses

[rolfyone]

Added SSZ support for validator balance responses using SszList.

Moved StateValidatorBalanceData into spec API schemas and added StateValidatorBalanceDataSchema.

Updated GET/POST validator balances endpoints to advertise application/octet-stream and return 406 Not Acceptable when needed.

Updated OpenAPI fixtures to include binary SSZ responses and a reusable ValidatorBalanceResponse schema.

Added tests for SSZ response serialization and 406 metadata handling.

Added StateValidatorBalances to the debug SSZ object types.

Cleaned up formatting/import issues found during the build check.

Addresses ethereum/beacon-APIs#622

Fixes #10934

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Medium Risk
Extends a widely used beacon REST API with new content negotiation and response shapes; JSON behavior should remain, but SSZ clients and metadata must stay aligned with the beacon-APIs spec.

Overview
Adds SSZ (application/octet-stream) responses for GET and POST /eth/v1/beacon/states/{state_id}/validator_balances, alongside existing JSON. Clients that negotiate SSZ get serialized SszList<StateValidatorBalanceData>; unsupported negotiation can return 406 Not Acceptable.

StateValidatorBalanceData is moved from api.migrated into spec.schemas.api as an SSZ container (StateValidatorBalanceDataSchema / ValidatorBalanceResponse), and ChainDataProvider now builds balances via SSZ_LIST_SCHEMA instead of a plain List.

OpenAPI fixtures gain application/octet-stream, a shared ValidatorBalanceResponse schema, and 406 responses. Tests cover SSZ metadata serialization and 406 handling. The debug CLI accepts .ssz input and adds StateValidatorBalances as an SSZ object type for pretty-printing.

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