Return 204 for producePayloadAttestationData when no block seen at slot

[rolfyone]

Implements compliance with beacon-APIs#612.

Previously, GET /eth/v1/validator/payload_attestation_data/{slot} returned 404 Not Found when no block had been seen at the requested slot. This was incorrect — a missing block is not an error condition; it is a normal signal to the PTC validator
that it should not cast a payload attestation.

The endpoint now returns 204 No Content in this case, matching the updated API specification. No changes were required on the client side (CreatePayloadAttestationDataRequest), as the base ResponseHandler already maps 204 to Optional.empty() via
its default noValueHandler.

Changes:

• GetPayloadAttestationData: replace 404 with 204 in both the OpenAPI metadata and the response handler
• GetPayloadAttestationDataTest: update tests to assert SC_NO_CONTENT and use verifyMetadataEmptyResponse

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

Low Risk
Targeted REST contract and client URL changes for one validator endpoint; behavior for success paths is unchanged aside from parameter location.

Overview
Aligns producePayloadAttestationData with beacon-APIs#612: GET /eth/v1/validator/payload_attestation_data now takes slot as a query parameter (not a path segment), and when no block has been seen for that slot the beacon node returns 204 No Content instead of 404, telling PTC validators not to cast a payload attestation.

GetPayloadAttestationData updates route metadata, OpenAPI responses (drops 404, documents 204), and maps Optional.empty() from the provider to SC_NO_CONTENT. The remote validator client CreatePayloadAttestationDataRequest and ValidatorApiMethod are updated to call the new URL shape with ?slot=…; existing ResponseHandler behavior for 204 → Optional.empty() is unchanged. Tests and the integration OpenAPI fixture are updated accordingly.

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