feat(v1): wrap the /api/v1 surface with RFC 9457 errors and cursor auto-pagination

[BSalaeddin]

Extends the SDK to Sendly's new `/api/v1` surface (33 operations) plus the two legacy list-subscription ops that landed in the released spec. Additive only — zero breaking changes; version 0.1.0 → 0.2.0.

What's new

• Resources: `campaigns` (10 ops incl. send/cancel/pause/resume/stats), `segments` (6), `workflows` (9, incl. executions), `analytics` (3), `usage` (1), v1 methods on `events` (`list`/`record`/`list_names`/`stats` — legacy `track` untouched), and legacy `lists.subscribe`/`lists.unsubscribe`.
• RFC 9457 errors: `/api/v1/*` failures are `application/problem+json`; they map onto the same `SendlyError` subclasses as the legacy envelope, with additive `request_id` and `field_errors` attributes. Detection trusts content-type and falls back to document shape.
• Cursor auto-pagination: `iter_*` generators on the 6 cursor-enveloped list ops (`{data, has_more, next_cursor}`), holding filters fixed per the API's cursor-fingerprint contract.
• Idempotency: `idempotency_key` on exactly the two spec-declared ops — `campaigns.create` and `campaigns.send` (the spec deliberately omits it elsewhere).
• v1 deletes return the real `{id, deleted}` body; `campaigns.send` accepts the optional `{scheduled_for}` body.

Verification

165 tests pass (84 new) · `ruff check` + `ruff format --check` clean · `mypy src` strict clean · contract test green with an empty not-implemented set (all 72 spec ops wrapped) · `python -m build` produces the 0.2.0 sdist+wheel. Webhook verifier untouched (zero diff). Release path: tag `v0.2.0` after merge — the tag workflow re-runs the full gate and publishes via OIDC trusted publishing.

Mirrors DevinoSolutions/sendly-js (same surface contract, reviewed for cross-language parity).

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