gh-158694: Slai Agent Runtime preserves empty Windows environment values - #159026

Summary

• Preserve empty Windows environment-variable values in the process environment and in child processes.
• Read the Windows process environment from GetEnvironmentStringsW() so os.reload_environ() also retains empty values.
• Add Windows regression coverage for os.environ[name] = '', child inheritance, and os.reload_environ().

Fixes gh-158694.

Problem

On Windows, the CRT call _wputenv(L"NAME=") removes NAME from the Process Environment Block. CPython intentionally uses _wputenv() to keep CRT consumers synchronized, but an empty assignment consequently disappears from child-process inheritance and is lost by os.reload_environ().

This preserves CRT synchronization and then explicitly writes the empty value to the process environment. The Windows environment reader now uses GetEnvironmentStringsW(), which is the authoritative source for the process environment and includes empty values. Hidden drive-current-directory entries remain excluded from os.environ.

Validation

• Built CPython x64 Debug on Windows: 0 warnings, 0 errors.
• test_os.test_os.EnvironTests.test_environ_empty_value: passed.
• Existing test_os.test_os.EnvironTests.test_putenv_unsetenv: passed.
• Manual built-interpreter validation confirmed an empty value remains visible before and after os.reload_environ() and is inherited by a child process.
• git diff --check: passed.

A full test_os run has one pre-existing local test_device_encoding failure in this non-console runner; I verified the same failure from the clean baseline, and all environment-focused tests passed.

Slai Agent Runtime context

Slai Agent Runtime encountered this while operating in a Windows automation environment that relies on child-process environment propagation. The developer authorized this contribution after the issue was reproduced and the fix was validated. This PR contains only CPython public-source changes and CPython-owned tests;