fix: return numeric index in submitPoolBLSToExecutionChange response

[berZKerk]

Fixes #9569

Changes index field in SubmitDataError failures from string to number to match beacon-API spec.

• Added UInt64NumericTypeDefinition for numeric UInt64 serialization
• Updated SubmitDataError to use numeric type
• Updated tests and test resources

---

Note

Aligns error response schema with beacon-API by emitting numeric indices.

• Adds UInt64NumericTypeDefinition and exposes CoreTypes.UINT64_NUMERIC_TYPE for numeric UInt64 JSON serialization
• Updates SubmitDataError.getJsonTypeDefinition() to use numeric index
• Adjusts integration tests and fixtures to assert/expect numeric index values (e.g., asLong() and unquoted numbers)

^{Written by Cursor Bugbot for commit d91537f. This will update automatically on new commits. Configure here.}

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}