Fix TestClient JSON null bodies

[cancaries]

Summary

• Distinguish omitted TestClient JSON bodies from explicit JSON null with an
  internal sentinel.
• Send b"null" and content-type: application/json when users pass
  json=None.
• Add regression coverage for request(), POST, PUT, PATCH, DELETE, and the
  body-style conflict checks.

Linked Issue

Fixes #118

Contributor Checklist

• The linked issue was marked accepted before I started.
• Nobody else was assigned or already working on the issue before I started.
• My branch is named issue_118.
• The PR is focused on one issue with no unrelated changes.
• Tests are added or updated when behavior changes.
• Docs are not updated because this is covered by TestClient behavior tests.
• Changelog is not updated because this repository does not appear to keep
  one for each focused bug fix.
• I listed the checks I ran below.

Checks Run

• python -m pytest tests/unit/test_test_client.py -q
• python -m ruff check src/quater/testing.py tests/unit/test_test_client.py
• python -m mypy src/quater/testing.py tests/unit/test_test_client.py

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates TestClient JSON body handling so callers can explicitly send a JSON null (via json=None) while still allowing “no JSON argument provided” to mean “no request body”.

Changes:

• Introduce a private sentinel (_JSON_NOT_GIVEN) to distinguish omitted json from explicitly provided None.
• Update _request_body ambiguity detection to account for the new sentinel.
• Add/extend unit tests to assert explicit json=None sends application/json with payload null, and to reject mixed body styles.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 9 comments.

File	Description
tests/unit/test_test_client.py	Adds coverage for explicit JSON null bodies and new ambiguity error cases.
src/quater/testing.py	Implements sentinel-based JSON argument handling so json=None becomes an intentional JSON body.

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[greptile-apps]

Greptile Summary

This PR fixes a long-standing ambiguity in TestClient where passing json=None was indistinguishable from omitting the json argument entirely — both resulted in an empty body with no Content-Type. The fix introduces a named private sentinel _JsonNotGiven so that explicit json=None now correctly serialises to b"null" with content-type: application/json.

• A _JsonNotGiven sentinel class (with __slots__ and __repr__) replaces the bare object() pattern; _JSON_NOT_GIVEN: Final is the singleton used as the default for json across all five verb methods and request().
• _request_body now uses identity checks (json is not _JSON_NOT_GIVEN) for both the conflict-detection and the serialisation branch, correctly propagating None as a JSON value.
• The integration test in test_cross_surface_parity.py is updated to use an explicit if "payload" in arguments guard, preventing the old arguments.get("payload") pattern from silently triggering the new "send null body" behaviour.

Confidence Score: 5/5

The change is safe to merge; it only adds a private sentinel class and updates default-value comparisons inside a testing helper, with no effect on production request handling.

All three changed files are testing infrastructure. The sentinel is private, identity-checked throughout, and Final-annotated to prevent reassignment. The conflict-detection and serialisation logic in _request_body was updated consistently, and the integration test was correctly adjusted so no existing call site accidentally sends a null JSON body. New tests cover the json=None round-trip for every affected verb as well as the conflict cases.

No files require special attention.

Important Files Changed

Filename	Overview
src/quater/testing.py	Introduces _JsonNotGiven sentinel, updates all json parameter defaults and _request_body logic; implementation is correct and well-contained.
tests/unit/test_test_client.py	Adds a regression test for json=None across all five HTTP methods and extends conflict-detection tests; coverage is thorough.
tests/integration/test_cross_surface_parity.py	Correctly replaces the arguments.get('payload') shortcut with an explicit key-presence check to avoid unintentionally sending a null JSON body.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["caller: client.post(path, json=X)"] --> B{"X is _JSON_NOT_GIVEN?"}
    B -- Yes --> C["No json body\n(omitted argument)"]
    B -- No --> D{"X is None?"}
    D -- Yes --> E["dumps_json(None) → b'null'\ncontent-type: application/json"]
    D -- No --> F["dumps_json(X) → bytes\ncontent-type: application/json"]
    C --> G{"content / data / files?"}
    G -- None --> H["body = b'', content-type = None"]
    G -- data+files --> I["multipart body"]
    G -- data only --> J["urlencoded body"]
    G -- content only --> K["raw content body"]
    E --> L["Request dispatched to app"]
    F --> L
    H --> L
    I --> L
    J --> L
    K --> L

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