Add contract test suite covering all resource modules

[aaronelliotross]

Summary

• Adds an HTTP-mocked (responses) pytest suite covering every public method across actionkit/ — connection/retry logic, base HttpMethods behavior, and all resource modules — replacing the previous two tests that only covered pure string/regex helpers (_path(), Users.id()).
• Adds responses as a dev dependency; shared fixtures live in tests/conftest.py / tests/urls.py.
• Wires pytest into .github/workflows/ci.yml so the suite is actually gated on PRs, not just present locally.
• The five pure HttpMethods subclasses with no added logic (signupactions, signuppages, genericpages, donationpages, userfields) don't get dedicated files — their behavior is already exercised by tests/test_httpmethods.py, plus a wiring smoke test in tests/test_actionkit.py.

Bugs found while writing this, filed separately (not fixed here)

Each is pinned with a regression test documenting current behavior, with an inline comment explaining why — not fixed as a side effect of adding tests:

• Connection converts body-bearing HTTP errors to ValidationError regardless of status code #15 — Connection._make_request converts any body-bearing HTTP error to ValidationError regardless of status code, likely making several except HTTPError: if status_code == ... branches elsewhere in the codebase dead code in production
• Transactions.create(order_id=...) always raises TypeError #16 — Transactions.create(order_id=...) always raises TypeError (unbound method call)
• Uploads.upload() polls forever with no timeout and leaks an open file handle #17 — Uploads.upload() polls forever with no timeout and leaks an open file handle
• Transactions.reverse()'s "already reversed" ValidationError check can never match #18 — Transactions.reverse()'s "already reversed" ValidationError check can never match
• Petitions.get() overrides HttpMethods.get() with an incompatible signature #19 — Petitions.get() overrides the base class with an incompatible signature

Test plan

• uv sync --group dev picks up responses without touching runtime dependencies
• uv run pytest — 199 passed, 0.23s locally
• Confirm the CI ci job actually runs and reports pytest results once this PR is open

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