Raise on failed system POST instead of swallowing it

[tipatterson-dev]

Fixes #42.

What was wrong

System.insert_self() assigned _resource_id only when res.ok, and did nothing otherwise — no raise, no log. Node.add_system(insert_resource=True) then returned normally, so the caller believed the system existed server-side. The failure surfaced much later as AttributeError: 'System' object has no attribute '_resource_id', typically from add_insert_datastream(), with no trace of the original HTTP error.

_resource_id was only created in System.__init__ when a resource_id kwarg was passed, so a from_resource()-built system didn't have the attribute at all — any pre-insert access was an AttributeError rather than a clean is None check.

Changes

resources/system.py — insert_self() raises on a non-ok response, with label, URN, status code, and response body. The message shape matches the four sibling add_insert_* methods that already raise.

resources/base.py — StreamableResource.__init__ sets self._resource_id = None unconditionally, fixing all three wrappers at once. System, Datastream, and ControlStream all assign their real id after super().__init__(), so nothing is clobbered. This also repairs System.retrieve_resource(), which already guarded on if self._resource_id is None and therefore died before reaching that check on a from_resource()-built system.

node.py — add_system docstring records that a failed POST propagates and the system is not attached.

docs/source/architecture/insertion.md — documents the raise.

Tests

Three added, 479 passing:

• test_insert_self_raises_on_failed_post — the raise carries both status code and response body
• test_resource_id_is_none_before_insert — None on both construction paths, and retrieve_resource() returns cleanly instead of raising
• test_add_system_does_not_attach_on_failed_insert — the issue's actual repro: raises, and leaves nothing in node.systems()

Compatibility

insert_self() now raises where it previously returned. Callers that relied on the silent return were, by construction, proceeding with a system that had no server-side id — the failure mode this fixes. The raise is a plain Exception, matching the rest of the insert path; #47 tracks promoting all five sites to a typed hierarchy.

Version bumped 0.5.2a1 → 0.5.3a1 (behavior fix, not routine iteration).

Follow-ups filed

Found in the same pass, not fixed here: #43, #44, #45, tracked with #47/#48/#49 under the umbrella #46.