Fix REST JSON KzgCommitment encoding/decoding to use hex strings per spec (#5522)

AllTests-mainnet.md

@@ -428,12 +428,13 @@ OK: 12/12 Fail: 0/12 Skip: 0/12
 + prune states                                                                               OK
 ```
 OK: 1/1 Fail: 0/1 Skip: 0/1
-## REST JSON decoding
+## REST JSON encoding and decoding
 ```diff
 + DenebSignedBlockContents                                                                   OK
++ KzgCommitment                                                                              OK
 + RestPublishedSignedBlockContents                                                           OK
 ```
-OK: 2/2 Fail: 0/2 Skip: 0/2
+OK: 3/3 Fail: 0/3 Skip: 0/3
 ## Remove keystore testing suite
 ```diff
 + Many remotes                                                                               OK
@@ -711,4 +712,4 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 OK: 9/9 Fail: 0/9 Skip: 0/9
 
 ---TOTAL---
-OK: 400/405 Fail: 0/405 Skip: 5/405
+OK: 401/406 Fail: 0/406 Skip: 5/406

beacon_chain/spec/eth2_apis/eth2_rest_serialization.nim

@@ -950,6 +950,21 @@ proc writeValue*(
 ) {.raises: [IOError].} =
   writeValue(writer, hexOriginal(distinctBase(value)))
 
+## KzgCommitment
+# https://github.com/ethereum/beacon-APIs/blob/d934a03187729635bef06ca7f3c067645c3eab15/types/primitive.yaml#L135-L140
+proc readValue*(reader: var JsonReader[RestJson], value: var KzgCommitment) {.
+     raises: [IOError, SerializationError].} =
+  try:
+    hexToByteArray(reader.readValue(string), distinctBase(value))
+  except ValueError:
+    raiseUnexpectedValue(reader,
+                         "KzgCommitment value should be a valid hex string")
+
+proc writeValue*(
+    writer: var JsonWriter[RestJson], value: KzgCommitment
+) {.raises: [IOError].} =
+  writeValue(writer, hexOriginal(distinctBase(value)))
+
 ## GraffitiBytes
 proc writeValue*(
     writer: var JsonWriter[RestJson], value: GraffitiBytes
@@ -1821,7 +1836,7 @@ proc readValue*(reader: var JsonReader[RestJson],
       value = RestPublishedSignedBlockContents(
         kind: ConsensusFork.Deneb,
         denebData: DenebSignedBlockContents(
-          # Constructed to be interally consistent
+          # Constructed to be internally consistent
           signed_block: signed_message.get().distinctBase.denebData,
           signed_blob_sidecars: signed_blob_sidecars.get()
         )

tests/test_rest_json_serialization.nim

@@ -203,7 +203,16 @@ const denebSignedContents = """
 }
 """
 
-suite "REST JSON decoding":
+# Can't be in same namespace as some other KZG-related fromHex overloads due to
+# https://github.com/nim-lang/Nim/issues/22861
+from stew/byteutils import hexToByteArray
+func fromHex(T: typedesc[KzgCommitment], s: string): T {.
+     raises: [ValueError].} =
+  var res: T
+  hexToByteArray(s, res)
+  res
+
+suite "REST JSON encoding and decoding":
   test "DenebSignedBlockContents":
     check: hash_tree_root(RestJson.decode(
       denebSignedContents, DenebSignedBlockContents, requireAllFields = true,
@@ -216,3 +225,33 @@ suite "REST JSON decoding":
       requireAllFields = true, allowUnknownFields = true).denebData) ==
         Eth2Digest.fromHex(
           "0x6b9fce0e35ee7af9b061f244706c4eda43c16e9dcc5b1cc817ed0671f49d16a8")
+
+  test "KzgCommitment":
+    let
+      zeroString =
+        "\"0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000\""
+      randString =
+        "\"0xe2822fdd03685968091c79b1f81d17ed646196c920baecf927a6abbe45cd2d930a692e85ff5d96ebe36d99a57c74d5cb\""
+      zeroKzgCommitment = KzgCommitment.fromHex(
+        "0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000")
+      randKzgCommitment = KzgCommitment.fromHex(
+        "0xe2822fdd03685968091c79b1f81d17ed646196c920baecf927a6abbe45cd2d930a692e85ff5d96ebe36d99a57c74d5cb")
+
+    check:
+      RestJson.decode(
+        zeroString, KzgCommitment, requireAllFields = true,
+          allowUnknownFields = true) == zeroKzgCommitment
+      RestJson.decode(
+        zeroString, KzgCommitment, requireAllFields = true,
+          allowUnknownFields = true) != randKzgCommitment
+      RestJson.decode(
+        randString, KzgCommitment, requireAllFields = true,
+          allowUnknownFields = true) != zeroKzgCommitment
+      RestJson.decode(
+        randString, KzgCommitment, requireAllFields = true,
+          allowUnknownFields = true) == randKzgCommitment
+
+      RestJson.encode(zeroKzgCommitment) == zeroString
+      RestJson.encode(zeroKzgCommitment) != randString
+      RestJson.encode(randKzgCommitment) != zeroString
+      RestJson.encode(randKzgCommitment) == randString