Perf/remote task mainthread receiver

[gabsow]

Note

Optimizes remote task handling and fixes refcount usage, with test harness hardening.

• Run REMOTE_TASK_FUNCTION_ID receiver on the Redis main thread: cluster.c defers callback via runCallbackOnUnblock and invokes it in MR_ClusterInnerCommunicationMsgUnblock; mr.c routes MR_RemoteTask to MR_RemoteTaskMainThread and logs enablement in MR_Init.
• Correct RedisModuleString refcount ops to use a real context (mr_staticCtx/ctx) instead of NULL across message/exec paths (HoldString/FreeString).
• Rust test module: add main-thread awareness and a global mutex around detached context; skip ThreadSafeContextLock on main thread to avoid crashes; record main thread in init.
• Tests: add pytests/requirements.txt (redis>=5); enhance run_tests.sh with redis-py version check, per-run/test timeouts, CI heartbeat, and diagnostics on timeout.

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