Fix passphrase quoting in decrypt_remote_file_to_string for Windows remotes

[gingeekrishna]

Motivation

Closes #69396

decrypt_remote_file_to_string in encryption_utils.py calls shell_quote_single() unconditionally. Single-quoting is Unix shell syntax and does not work on Windows CMD/PowerShell, so any call targeting a Windows remote host produced a malformed openssl command.

The issue author notes: "[Note that the decrypt_remote_file function in tpt_util.py does cater for Windows.]" — that function already handles this correctly via get_remote_os().

Changes

Apply the same OS-detection pattern already used in tpt_util.py:decrypt_remote_file:

• Detect the remote OS via get_remote_os(ssh_client)
• Windows: wrap password in double quotes, escape embedded " as ""
• Unix/Linux: keep existing shell_quote_single() behaviour (no functional change)

# Before (Unix only)
quoted_password = shell_quote_single(password)

# After (OS-aware)
remote_os = get_remote_os(ssh_client)
if remote_os == "windows":
    quoted_password = '"' + password.replace('"', '""') + '"'
else:
    quoted_password = shell_quote_single(password)

Tests

• Updated test_decrypt_remote_file_to_string to mock get_remote_os returning "unix" (no behaviour change for Unix path)
• Added test_decrypt_remote_file_to_string_windows to assert double-quote escaping is used on Windows remotes, including embedded " characters in the password

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