Sync OpenAPI: add clone and artifact endpoints, replace CostModel

Regenerate the client against the upstream v0.4 spec (issue #52).

- Add clone_job (POST /jobs/{UUID}/clone) and CloneJobPayload.
- Replace get_compiled_file (GET /jobs/{UUID}/circuits/{lang}) with
  get_job_artifact (GET /jobs/{UUID}/artifacts/{artifactId}); the response
  body is opaque, so only the *_detailed callables are generated.
- Replace CostModel with ApiCostModel ("QCT" / "2QGE_operations").
- Add Backend.supported_gates/supported_native_gates/supported_error_mitigations.
- Add estimate_job_cost fields: estimated_quantum_compute_time_us, plus
  rate_information.qct_cost_cents and rate_type; cost fields now nullable.
- Update tests, AGENTS.md, and CHANGELOG accordingly.

Author: splch

AGENTS.md

@@ -75,12 +75,12 @@ Every endpoint module exposes four callables: `sync`, `sync_detailed`, `asyncio`
 
 ```python
 from ionq_core import IonQClient
-from ionq_core.api.default import create_job, get_job, get_compiled_file, get_jobs
+from ionq_core.api.default import create_job, get_job, get_variant_probabilities, get_jobs
 from ionq_core.models.circuit_job_creation_payload import CircuitJobCreationPayload
 
 client = IonQClient()                                          # reads IONQ_API_KEY
 get_job.sync(uuid, client=client)                              # one path param
-get_compiled_file.sync(uuid, lang, client=client)              # multiple path params
+get_variant_probabilities.sync(uuid, variant_id, client=client)  # multiple path params
 get_jobs.sync(client=client, status="completed", limit=10)     # query only
 create_job.sync(client=client, body=payload)                   # body only
 ```

CHANGELOG.md

@@ -9,12 +9,21 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 ### Added
 
 - `QctrlQaoaJobCreationPayload` and `QctrlQaoaJobInput` for submitting Q-CTRL QAOA maxcut combinatorial-optimization jobs via `create_job`. The `create_job` body union now also accepts `QctrlQaoaJobCreationPayload`.
-- `cost_model` optional field on `BaseJob`, `GetCircuitJobResponse`, and `GetJobResponse`, typed as `CostModel` (`"quantum_compute_time"` or `"execution_time"`).
+- `cost_model` optional field on `BaseJob`, `GetCircuitJobResponse`, and `GetJobResponse`, typed as `ApiCostModel` (`"QCT"` or `"2QGE_operations"`).
+- `clone_job` endpoint (`POST /jobs/{UUID}/clone`) and its `CloneJobPayload` model for resubmitting an existing job with optional overrides.
+- `get_job_artifact` endpoint (`GET /jobs/{UUID}/artifacts/{artifactId}`) for downloading job artifacts by id. The response body is opaque, so only the `sync_detailed` / `asyncio_detailed` callables are generated; read the bytes off `Response.content`.
+- `Backend` now exposes `supported_gates`, `supported_native_gates`, and `supported_error_mitigations`.
+- `estimate_job_cost` response gained `estimated_quantum_compute_time_us`, and its `rate_information` gained `qct_cost_cents` and `rate_type` (`"qct"` or `"2qge"`). Its `cost_1q_gate`, `cost_2q_gate`, and `job_cost_minimum` rate fields are now nullable.
 
 ### Changed
 
 - `NativeCircuitInput.qubits` and `JsonMultiCircuitInput.qubits` are now `int | Unset` (previously `float | Unset`), matching upstream's tightening to `format: int32, minimum: 1`. `QisCircuitInput.qubits` already had this type locally via the OpenAPI overlay; that overlay action has been removed now that upstream is correct natively.
 
+### Removed
+
+- `get_compiled_file` endpoint (`GET /jobs/{UUID}/circuits/{lang}`) and its `GetCompiledFileLang` enum, removed upstream in favor of `get_job_artifact`. Compiled circuits are now fetched as artifacts by id rather than by `lang` (`"native"` / `"qasm3"`).
+- `CostModel` model, replaced by `ApiCostModel`.
+
 ## [0.1.1] - 2026-04-30
 
 ### Changed