Fix MPI-Checker false positive in MPIRequest::wait()

[Copilot]

Ticket

N/A

Problem description

Clang Static Analyzer's optin.mpi.MPI-Checker warns at line 128: "Request has no matching nonblocking call". This is a false positive—the analyzer cannot track MPI_Request objects through class members. The request is properly initialized via MPI_Isend/MPI_Irecv in factory methods before being passed to MPIRequest constructor.

What's changed

Suppression: Added NOLINT(clang-analyzer-optin.mpi.MPI-Checker) to MPI_Wait and MPI_Test calls.

Defensive checks: Added if (done_) guards in wait() and test() to prevent double-wait/test. After MPI_Wait completes, req_ is set to MPI_REQUEST_NULL by MPI; subsequent calls return sentinel values.

Documentation:

• Explained analyzer limitation and why warning is false positive
• Documented constructor precondition (requires valid MPI_Request from nonblocking call)
• Clarified sentinel return values (MPI_ANY_SOURCE/MPI_ANY_TAG indicate no-op, not wildcards)
• Noted callers should use active() before calling wait()

Impact: 18 lines added (mostly comments), 2 NOLINT suppressions. No functional changes to correct usage.

Checklist

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• New/Existing tests provide coverage for changes

Model tests

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  • models-mandatory preset (runs: Device perf regressions and Frequent model and ttnn tests)
  • models-extended preset (runs: the mandatory tests, plus Demo and Model perf tests)
  • other selection - specify runs

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  • models-mandatory preset (runs: Unit tests)
  • models-extended preset (runs: the mandatory tests, plus Demo and Model perf tests)
  • other selection - specify runs

• 

  • models-mandatory preset (runs: Quick tests)
  • models-extended preset (runs: the mandatory tests, plus Demo and Model perf tests)
  • other selection - specify runs

Original prompt

Fix this Clang Static Analyzer issue:

• optin.mpi.MPI-Checker in tt_metal/distributed/multihost/mpi_distributed_context.cpp:128 — Request has no matching nonblocking call.

Verify it's a real issue, apply minimal fix, follow coding standards.

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