Add QPU device_call host-dispatch path for realtime QEC decoding

[vedika-saravanan]

Description

This PR adds a CUDA-Q device_call host-dispatch path for realtime QEC decoding.

The new path allows a lowered QPU kernel call to invoke QEC decoder operations through CUDA-Q's --cudaq-device-call=host-dispatch runtime. The HOST_CALL handlers forward into the existing CUDA-QX realtime decoder host API, enabling an end-to-end QPU-kernel-to-decoder flow for CPU decoders such as vanilla PyMatching.

Changes include:

• Add cudaq-qec-realtime-decoding-host-dispatch, a DeviceCallService plugin for QEC realtime decoder calls.
• Add QPU-side wrappers for enqueue_syndromes, get_corrections, and reset_decoder using cudaq::device_call(0, ...).
• Register HOST_CALL entries for packed uint64_t enqueue/get/reset decoder RPCs.
• Add an end-to-end PyMatching test that runs a QPU kernel, enqueues syndromes, retrieves corrections, and verifies reset behavior through host-dispatch.
• Wire the new optional target/test behind CUDA-Q realtime/device_call availability.

Runtime / performance impact

N/A for existing paths. The new host-dispatch target is optional and only built when CUDA-Q realtime/device_call dependencies are available.

Self-review checklist

Please confirm each item before requesting review. Check [x] or strike
through and explain.

Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
• CI logs reviewed; no unexplained warnings or errors.
• Full CI has been run.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing.
• Negative tests added where exceptions are expected.
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness.
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs.
• User-visible behavior changes have public docs, or a follow-up is
  tracked.
  Docs: N/A. This PR adds internal host-dispatch wiring and an integration test; it does not introduce a new public API or documented user workflow.

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.