fix(skill_toolset): enforce utf-8 encoding when materializing skill files on Windows

[mc-marcocheng]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Bug: UnicodeEncodeError on Windows when materializing skill resources in _SkillScriptCodeExecutor #5819
• Related: Bug: UnicodeEncodeError on Windows when materializing skill resources in _SkillScriptCodeExecutor #5819

2. Or, if no issue exists, describe the change:

Problem:
When running the ADK on Windows, executing a skill script via _SkillScriptCodeExecutor fails with a UnicodeEncodeError if the skill's resources (references, assets, or scripts) contain non-ASCII characters. The generated wrapper script writes these files without specifying an encoding, causing Windows to fall back to its system locale encoding (e.g., cp1252).

Solution:
By explicitly setting encoding='utf-8' when mode == 'w' in the generated wrapper script, we ensure that text files are correctly written regardless of the system's default locale encoding. Binary assets (mode == 'wb') continue to be handled properly without an encoding argument.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of passed pytest results:

tests\unittests\tools\test_skill_toolset.py ................................................................ [ 67%]
...............................                                                                              [100%]
========================================= 95 passed, 6 warnings in 15.68s =========================================

Manual End-to-End (E2E) Tests:

• Reproduced the issue manually on Windows using a custom Python script that invokes _build_wrapper_code with Chinese characters (你好，世界) and executed the generated script.
• Verified that applying the fix resolves the UnicodeEncodeError during manual execution.
• Wrote an integration test (test_integration_python_unicode_materialization) that acts as an automated E2E test by using the real UnsafeLocalCodeExecutor to materialize and run a script reading the Unicode content.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

N/A

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