feat: Add RunSkillScriptTool to SkillToolset

Introduces RunSkillScriptTool to execute scripts located in a skill's scripts/ directory.

The execution logic is isolated within a dedicated SkillScriptCodeExecutor wrapper instantiated by RunSkillScriptTool. This wrapper manages script materialization in a temporary directory and executes Python (via runpy) or Shell scripts (returning standard output or JSON-encoded envelopes).

This isolation eliminates the need to modify the underlying `BaseCodeExecutor` interface or implementations (`unsafe_local_code_executor`, etc.) to support working directories or file paths.

Co-authored-by: Haiyuan Cao <haiyuan@google.com>
PiperOrigin-RevId: 875012237

Author: haiyuan-eng-google