Fix Block model to match server: nest header with camelCase keys

The block endpoint returns block id / prev / timestamp nested under a
"header" object with camelCase keys (blockId, dcId, prevId, prevProof,
timestamp) and a proof of just {proof}. The previous flat snake_case
Block fields never matched the response and always deserialized empty.
Add a BlockHeader dataclass, nest it in Block, make proof.scheme
optional, and cover it with a unit test. Verified live against a dev
chain.

tests/test_client.py

@@ -13,6 +13,7 @@ import json
 import pytest
 
 from prime_sdk import (
+    Block,
     DragonchainAPIError,
     DragonchainSDK,
     SmartContractCreateRequest,
@@ -163,6 +164,33 @@ def test_transaction_from_dict_nested():
     assert txn.payload == "{}"
 
 
+def test_block_from_dict_nested_header():
+    # Real server shape: id/prev/timestamp nested under "header" with camelCase
+    # keys; proof carries only "proof" on a trust-scheme chain.
+    raw = {
+        "version": "1",
+        "header": {
+            "blockId": "69569983",
+            "dcId": "chain-xyz",
+            "prevId": "69186326",
+            "prevProof": "MEUCIQ...",
+            "timestamp": "1780088135",
+        },
+        "proof": {"proof": "MEQCIF..."},
+        "transactions": ["{...}", "{...}", "{...}"],
+    }
+    blk = Block.from_dict(raw)
+    assert blk.version == "1"
+    assert blk.header.block_id == "69569983"
+    assert blk.header.dc_id == "chain-xyz"
+    assert blk.header.prev_id == "69186326"
+    assert blk.header.prev_proof == "MEUCIQ..."
+    assert blk.header.timestamp == "1780088135"
+    assert blk.proof.proof == "MEQCIF..."
+    assert blk.proof.scheme == ""  # absent on trust-scheme chain
+    assert len(blk.transactions) == 3
+
+
 def test_contract_create_request_omitempty():
     req = SmartContractCreateRequest(
         environment="python3.8",