Implements docs/contracts/issues/2.contract.md. Two functions
(create_session, list_sessions), two frozen dataclasses (SessionInfo,
SessionPage), three exception types (AgentNotFound, InvalidCursor,
SessionApiFailed). 19 contract-listed tests cover every TESTS:
entry verbatim per the tracer-bullet vertical-slice ordering.
SessionInfo uses one shape across both endpoints with origin-
conditional defaults per INV-001 (create) and INV-002 (list). create-
origin always sets list-only fields to (name=None, archived=False,
tags=[]); list-origin reads them from the response item with
absent/null treated as those same defaults — keeps the dataclass
uniform without forcing callers to handle two types.
Spotted an internal-inconsistency in the contract at TDD start —
POST-003 and happy_create's test description still said "archived
is None, tags is None" while the freshly-applied Volva amendment
had moved INV-001 to (archived=False, tags=[]). Fixed in-place
before writing any tests so the spec stayed coherent.
SessionApiFailed.body truncates to <= 1024 bytes at construction,
matching the SseConnectFailed / CancelFailed precedent from issue #1.
No code shared with sse_client.py (convention-dependency only per
issue #2's dependencies: block). 62 tests GREEN total (42 sse_client
+ 19 sessions + 1 boundary smoke). Ruff clean.
No refactor pass — the two functions are ~25 LOC each with distinct
error-routing branches that don't naturally share more than they
already do.
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