feat: unsafe local code executor

[kalenkevich]

Link to Issue or Description of Change

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

• Related: Support for skills in the js adk #155

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

Problem:
The ADK lacked a concrete implementation for executing code locally in a Node.js environment. While BaseCodeExecutor provided the structure, there was no ready-to-use executor for running JavaScript or TypeScript code blocks locally. Additionally, BaseCodeExecutor was missing default delimiters for javascript and typescript code blocks.

Solution:

1. Implemented UnsafeLocalCodeExecutor in core/src/code_executors/unsafe_local_code_executor.ts. This executor:
   • Writes the provided code to a temporary .js file.
   • Spawns a child process using Node.js (by default) to execute the file.
   • Captures stdout and stderr.
   • Supports a configurable timeout to prevent hanging executions (defaults to 30s).
   • Cleans up the temporary file after execution.
2. Updated BaseCodeExecutor in core/src/code_executors/base_code_executor.ts to include javascript and typescript in codeBlockDelimiters and updated documentation examples to use JavaScript.
3. Exported UnsafeLocalCodeExecutor from core/src/index.ts.
4. Added Unit Tests:
   • core/test/code_executors/base_code_executor_test.ts to verify default values and type guards.
   • core/test/code_executors/unsafe_local_code_executor_test.ts to verify code execution, stderr capture, and timeout behavior.

Testing Plan

Unit Tests:

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

Manual End-to-End (E2E) Tests:
No manual E2E tests were performed as the unit tests cover the execution logic by spawning actual Node.js processes.

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.