feat(tasks): enforce unassigned tasks start Unclaimed on create

[lkubie]

What

client.tasks.create / create_many now enforce the platform's task lifecycle rule client-side: a task created without assigned_to always starts Unclaimed. A missing state is defaulted to Unclaimed in the outgoing payload, and ownership states (Not Started, In Progress) are rejected with a clear AlbertException before any request is sent. The invariant ("a task cannot be In Progress and unassigned") is documented on TaskState, BaseTask.state / BaseTask.assigned_to, and the create / create_many / update docstrings (which feed the public docs site).

Why

SDK counterpart to EX-2918 / https://github.com/MoleculeEngineering/api-task/pull/486 (requested by Lenore Kubie). The API now defaults unassigned task creates to Unclaimed because stateless tasks break the UI and cannot be repaired via PATCH (state transitions require the current state). Enforcing the same contract in the SDK means callers get a deterministic payload and a clear client-side error instead of relying on server-side defaulting. Linear: EX-2934.

How

• New pure helper resolve_unassigned_task_state in src/albert/utils/tasks.py, mirroring the API's taskState.helper.js: None -> UNCLAIMED, NOT_STARTED / IN_PROGRESS -> raise, explicit UNCLAIMED / terminal states (COMPLETED, CLOSED, CANCELLED) pass through unchanged for ingestion flows (EX-2401).
• Applied in TaskCollection.create and create_many only when assigned_to is None, by setting state on the serialized payload. create_with_combinations delegates to create, so it is covered too.
• update is intentionally not blocked: the platform auto-assigns the calling user when an unclaimed task is moved forward (FTR-257), which is now documented in the update Notes instead.

Testing

• 7 new unit tests in tests/unit/utils/test_tasks.py covering the default, both rejected ownership states, and the pass-through table.
• uv run pytest tests/unit: 1278 passed, 2 xfailed. ruff format / ruff check: clean.
• Integration tests not run (require live API credentials); the change is offline payload-assembly logic covered by unit tests.

Cake session: https://agents.ai.albertinventdev.com/sessions/e26da368-3c7e-4ac2-8ab2-53b852318ce5

[claude]

Looks good - no bugs or correctness issues found.