tests/interaction: pre-stage spec_version=2026-07-28 arm_exclusions

43 requirements whose spec section survives in 2026-07-28 but whose
test bodies cannot run on the 2026 client path are annotated with
arm_exclusions=(ArmExclusion(reason=..., spec_version="2026-07-28"),):

- server-initiated-request (27): sampling/elicitation/roots tests that
  drive ctx.sample/elicit/list_roots, which issue JSON-RPC server-to-
  client requests. In 2026 those become MRTR input_required payloads;
  re-admit when MRTR adapters land.
- requires-session (7): list-changed notifications, cross-POST
  cancellation, late-progress -- all need persisted per-session state
  the 2026 per-request model does not have.
- asserts-legacy-handshake (5): test bodies that read InitializeResult
  fields or assert _meta shape that 2026's per-request envelope changes.
- legacy-only-vocabulary (4): InitializeResult.capabilities flags and
  unsolicited logging that server/discover does not carry.

36 of the 43 extend an existing stateless arm_exclusions tuple; 7 are
new single-member tuples. Inert today; when SPEC_VERSIONS gains
2026-07-28 these entries automatically drop their 2026 cells.

504 connect cells unchanged; 100% coverage; pyright/ruff clean.

Author: maxisbey