fix: POSIX line-continuation in process_command splitting

[yndu13]

Summary

• Treat backslash-newline as POSIX line continuation (both removed) in CommandLineUtils.split, outside quotes and inside double quotes, matching shlex semantics (follow-up to fix: quote-aware External process_command parsing #104).
• Add test cases for both continuation positions.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.22%. Comparing base (b3a2335) to head (ae5e25c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...com/aliyun/credentials/utils/CommandLineUtils.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #105      +/-   ##
============================================
+ Coverage     87.20%   87.22%   +0.02%     
- Complexity      838      840       +2     
============================================
  Files            53       53              
  Lines          3102     3108       +6     
  Branches        306      308       +2     
============================================
+ Hits           2705     2711       +6     
  Misses          289      289              
  Partials        108      108              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.