Introduce programmatic popen and run execution layers with distributed process controllers

[dmitry-lipetsk]

Description

This PR refactors the core process execution layer in testgres.os_ops to transition from the legacy exec_command to a modern, decoupled abstraction model utilizing popen and run. This layer guarantees cross-platform uniformity, preventing descriptor leaks and handling complex environmental differences (such as standard library variations in Alpine or specific hardware constraints).

Core Enhancements

1. Uniform Interface Layer (OsProcessController & OsCommandResult)

   • Implemented abstract and runtime process controllers (LocalProcessController, RemoteProcessController) to provide full API symmetry with standard subprocess.Popen.
   • Integrated explicit runtime verification checks (io.IOBase) for all standard I/O pipes, descriptors, and streams.

2. Robust Multi-stage Remote Handshake Mechanism

   • Redesigned RemoteOperations::popen around a multi-stage POSIX-compliant script wrapper.
   • Uses precise time.monotonic() loops to execute state handshakes via remote transient tracking files (pid_file, rc_file).
   • Eliminates historical network transport 255 exit status side-effects, ensuring exact signal propagation (137 for SIGKILL, 143 for SIGTERM).

3. Prevention of Deadlocks and Resource Leaks

   • Integrated fully isolated contextual managers (__enter__ / __exit__) across controllers to automate cleanup procedures.
   • Implemented safety-first deterministic __init__ sequences combined with defensive __del__ destructors to force-terminate dangling SSH client pipes and avoid active zombie nodes.

4. Emergency Configuration Overrides (OsOperationStaticConfig)

   • Introduced a centralized, strict static configuration layer initialized at module-import time to avoid repetitive runtime os.environ overhead.
   • Added the TESTGRES_OS_OPS_CFG__REMOTE_OPS__POPEN__HANDSHAKE_TIMEOUT environment variable option (defaulting to 10.0s, with a valid range up to 4 hours) to prevent unexpected test flakes under high-density infrastructure loads (e.g., Elbrus/e2k compilation farms).
   • Embedded strict type, value boundary validations, and verbose _debug_print diagnostics to prevent silent misconfigurations in CI/CD environments.

Testing Suite Summary

• Expanded test matrix mapping over multiple runtime scenarios covering complex argument matrices (text, encoding, shell, input, streams).
• Successfully validated across multi-node execution nodes (including Ubuntu, Alt Linux, Rocky Linux, Astra Linux, and Alpine musl).

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What's Next

This PR lays the public contract for user space execution. Internal utilities (e.g., read_binary, mkstemp, remove_file) will be isolated onto private _transport_popen and _transport_run wheels in a follow-up patch to cleanly decouple internal system tasks from user pipelines.