Fix log.file configuration in v4.1 multi-provider format

Problem Summary

In DDNS v4.1, the log.file configuration was ignored when using the new providers array format with multiple providers. Console logging worked normally and domain updates proceeded, but logs were not written to the specified file.

User Report: "之前4.02版本这么写，log file记录都是正常的。更新到了4.1 log is正常显示，域名更新也是正常的，但是file里面不记录了"

Root Cause Analysis

The _setup_logging function in ddns/config/__init__.py had a condition that only used log settings from the first config when len(all_json_configs) == 1:

# Buggy code:
json_config = all_json_configs[0] if len(all_json_configs) == 1 else {}

When using the v4.1 providers format with multiple providers, len(all_json_configs) > 1, causing an empty dict {} to be used, which lost all log configuration including log.file.

The Fix

Changed one line to always use the first config for log settings:

# Fixed code:
json_config = all_json_configs[0] if all_json_configs else {}

Why this is safe: In v4.1 format, all provider configs inherit the same global log settings from the parent configuration. The log configuration is identical across all providers, so using the first config is correct.

Changes Made

• Fixed ddns/config/__init__.py: Changed condition in _setup_logging function (1 line)
• Added test_v41_multi_provider_log_config regression test in tests/test_config_schema_v4_1.py
• Refactored Test imports moved to module level following Python conventions
• Fixed Python 2.7 compatibility in tests (assertIn for log_level check)
• Fixed Linting issues (whitespace)
• Verified all tests pass in both Python 2.7 and 3.x
• Tested end-to-end with actual log file creation and writing

Test Results

✅ All tests pass (Python 2.7 and 3.x compatible)

• Single provider v4.1 format: Works correctly ✓
• Multiple providers v4.1 format: Now fixed ✓
• V4.0 format compatibility: Maintained ✓
• End-to-end log file creation: Verified ✓
• Linting: All whitespace issues resolved ✓
• Python 2.7: All tests pass ✓

Impact

• No breaking changes: Fully backward compatible
• Minimal change: Only 1 line changed in production code
• Solves the issue: Log files now work correctly with multiple providers in v4.1 format

Example Fixed Configuration

{
  "$schema": "https://ddns.newfuture.cc/schema/v4.1.json",
  "log": {
    "level": "INFO",
    "file": "/var/log/ddns.log",
    "format": "%(asctime)s %(levelname)s [%(filename)s:%(lineno)d]: %(message)s",
    "datefmt": "%Y-%m-%d %H:%M:%S"
  },
  "providers": [
    {"provider": "cloudflare", "id": "[email protected]", "token": "token1", "ipv4": ["domain1.com"]},
    {"provider": "dnspod", "id": "[email protected]", "token": "token2", "ipv4": ["domain2.com"]}
  ]
}

Now logs are correctly written to /var/log/ddns.log even with multiple providers! 🎉

Fixes #563

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