engine: add -38005 error codes in engine_newPayloadV2, engine_getPayloadV2 and engine_forkchoiceUpdatedV2 openrpc methods

[swastik7805]

Closes #776

Fix: Add -38005 Error Code to V2 Engine Method Specs

Problem

engine_forkchoiceUpdatedV2, engine_newPayloadV2, and engine_getPayloadV2 are missing the -38005 (Unsupported fork) error declaration in their OpenRPC YAML definitions.

As Per cancun.md,
these methods must return -38005 when the payload or payloadAttributes timestamp is >= the Cancun activation timestamp. The Markdown spec is correct but the YAML schema does not reflect this.

Changes

• forkchoice.yaml — added -38005 to engine_forkchoiceUpdatedV2
• payload.yaml — added -38005 to engine_newPayloadV2
• payload.yaml — added -38005 to engine_getPayloadV2

Fix

errors:
  - code: -38005
    message: Unsupported fork

References

• cancun.md — Update the methods of previous forks
• forkchoice.yaml#L45
• payload.yaml#L64
• payload.yaml#L316

@MysticRyuujin please review