fix(api): Circles.list() unwraps the data envelope; Circles.new() sends tag/parents

[nycomp]

Fixes #55 (both defects).

Changes

• Circles.list() unwraps ["data"] — the API's list endpoints return a {"data": [...]} envelope (assignments, submissions, timetable all do); the client looked up a "circles" key that is never sent.
• Circles.new() now sends tag and parents in the POST payload (parents only when provided) — previously both parameters were silently dropped, so created circles never carried them.
• description now defaults to "" to match the API, where it is optional server-side (fix(circles): @meta filter in list(), 409 on missing-circle delete (#760) campus#763 made it default "").
• New tests/unit/test_circles.py: envelope unwrap + payload-carries-tag/parents regressions, in the existing mock-client style.

Notes

• The CircleMembers sub-resource defects (wrong endpoints, flat-mapping unwrap) are not touched here — they are fix(api): CircleMembers add/remove target the wrong endpoints; members.list() unwraps a key the API does not send #42.
• Server-side contracts were already fixed and deployed on dev in fix(circles): @meta filter in list(), 409 on missing-circle delete (#760) campus#763/#764 (circles phases 1–2, circles: current → desired delta and close-out plan (umbrella for #501) campus#759/#762); this PR is the client half. A dev E2E sweep against the live API follows under circles phase 3: client envelope fix (campus-api-python#55) + end-to-end dev verification + close-out campus#762.

Verification

• poetry run pytest: 118 passed (4 new).
• ruff check on changed files: clean.

[nycomp]

Dev E2E verification (nyjc-computing/campus#762 sweep), driven end-to-end through this branch's client against https://campusapi-development.up.railway.app (deploy a5bf25e, campus phases 1–2 live) — 8/8 checks passed:

[PASS] list-200-no-meta-leak: 2 circles: uid-circle-4c2094d0(root), uid-circle-e4f032fc(admin)
[PASS] new-carries-tag-parents: tag='project' root.members[uid-circle-ace50b83]=10
[PASS] patch-name-only-200: name='e2e-p3-a-renamed-e20c10a3'
[PASS] dup-name-create-409: got 409 (want 409): Circle name already exists
[PASS] dup-name-rename-409: got 409 (want 409): Circle name already exists
[PASS] root-plus-parents-409: got 409 (want 409): Root circle cannot have parents
[PASS] patch-missing-409: got 409 (want 409): Circle not found
[PASS] delete-missing-409: got 409 (want 409): Circle not found

Notes:

• new-carries-tag-parents verified parents as a membership edge in the parent circle's members dict (root.members[scratch_id] == 10) — that is how the server records parentage; the circle record itself has no parents field.
• All scratch circles deleted after the sweep; dev circles collection back to root + campus-admin.