feat(executors): add IonQ Direct API executor

[Adithyaphani]

Closes #1.

Summary

• Add IonQExecutor / IonQExecutorConfig for direct IonQ v0.3 REST API
  submission (no AWS Braket routing required).
• Convert circuits via circuit.to_qiskit() -> OpenQASM 2.0 -> IonQ "qasm"
  input format.
• Poll job status; map IonQ probability results to ExecutionResult.counts
  (bitstring counts, matching BraketExecutor's output shape).
• Map IonQ job failures (failure.error / failure.code) to a descriptive
  RuntimeError.
• Map IonQ backend status (available/running/calibrating/unavailable/
  reserved/offline) to DeviceStatus per CONTRIBUTING.md §2 (device-level,
  not job-level).
• Register "IonQ Direct" in ExecutorFactory, export from
  marqov/executors/__init__.py.
• Update README's "Supported Backends" table to mark IonQ Direct as
  available.
• Unit tests with a mocked IonQ client (15 tests) covering: successful
  execution, polling to completion, job failure handling, cancel
  (success/failure), backend status mapping for all documented states,
  probability->counts bit-ordering, factory registration, and missing-API-key
  error handling.

Here's the architecture, in two diagrams, IonQExecutor sits in the codebase:

Now the request/data flow inside execute():

Design note

The issue references the ionq PyPI package, which is currently pre-release
(0.0.0a15) and doesn't yet cover job polling, results retrieval, cancellation,
or backend status -- all required by the acceptance criteria. Rather than
depend on an unstable alpha SDK, this implements a small injectable REST
client (_IonQRestClient) against IonQ's documented v0.3 API
(https://docs.ionq.com/guides/direct-api-submission). No new dependency is
required -- requests is already a transitive dependency (via azure-core,
ibm-cloud-sdk-core, qiskit-ibm-runtime). The client is a thin, swappable
layer if the official SDK matures.

Verification

• ruff check marqov/executors/ionq.py marqov/executors/factory.py marqov/executors/__init__.py tests/test_ionq_executor.py -- all checks passed
• pytest tests/test_ionq_executor.py -v -- 15 passed
• pytest tests/ -q -- 283 passed, 13 skipped, 0 failed (after excluding 3
  pre-existing failures, see note below)
• Submitted a real request to https://api.ionq.co/v0.3/jobs (with an
  invalid placeholder key) and confirmed the client correctly surfaces IonQ's
  401 Unauthorized via requests.HTTPError -- the request/auth/error-
  propagation path is exercised against the live API, not just mocks.

Pre-existing test failures (unrelated)

tests/test_activity_heartbeat.py, tests/test_decorators.py, and
tests/test_temporal_activities.py fail with RecursionError: Stack overflow
(cloudpickle serialization) on Python 3.14 in this environment. Confirmed via
git stash that these fail identically on main without this change -- a
pre-existing environment issue unrelated to this PR.

AI disclosure

Claude helped draft the initial IonQExecutor implementation and tests. I reviewed every method, ran the full suite, debugged and isolated three pre-existing environment failures unrelated to this change (confirmed via git stash), verified the dependency claims, checked the live API's error response, and updated the README.

[Adithyaphani]

@ddri looking forward to your review and response on the opened PR , happy to make further changes if needed.

[Adithyaphani]

@ddri looking forward to your review & response, and happy to make further changes if needed.