fix(headers): use json.dumps for HMAC body serialization (closes #4)

[Ccheh]

Bug

client.py:418 stringifies the request body with str() before HMAC:

​python if body is not None: body = str(body) headers = self.builder_config.generate_builder_headers(method, request_path, body) ​

str({"a": "b"}) → {'a': 'b'} (Python repr, single quotes, not JSON).

Why it breaks

The wire body is sent via requests.request(..., json=body) in
http_helpers/helpers.py, which serializes through json.dumps(...)
(double quotes). HMAC signs one string; the server verifies a different
one.

For BuilderConfig.remote_builder_config, the Node/Express
builder-signing-server fails JSON.parse on the Python repr and
returns 400 — this is what #4 reports.

Fix

​```diff

•    body = str(body)
  

•    body = json.dumps(body)
  

​```

json.dumps with default separators matches requests' wire format
exactly, so the signed bytes and transmitted bytes are identical.

Test

Adds tests/test_client_headers.py with two cases:

• the body handed to BuilderConfig.generate_builder_headers is valid
  JSON, equals json.dumps(input_dict), and contains no single quotes
• a None body is passed through unchanged

Full suite: 22/22 passing.

Compatibility

No public API change. body was already a dict at the call site
(_post_request); only the local rebinding inside
_generate_builder_headers changes.

Closes #4

---

Note

Medium Risk
Touches request-signing header generation used for authenticated relayer calls; while the change is small, mismatched serialization could still break signature verification across clients/servers if expectations differ.

Overview
Fixes builder header/HMAC signing to serialize request bodies with json.dumps (instead of Python str(dict)), aligning the signed bytes with the JSON payload sent via requests(..., json=body).

Adds tests/test_client_headers.py to assert the forwarded body is valid JSON, matches json.dumps(...) exactly, and that None bodies are passed through unchanged.

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