fix(vlm): rollback ResponseAPI to Chat Completions, keep tool calls

[chenjw]

• Replaced Responses API with Chat Completions API in volcengine_vlm.py
• Removed prompt caching features (LRUCache, response_id prefix caching)
• Retained tool calling support via _parse_tool_calls and _build_vlm_response
• Fixed tool result message format in memory/tools.py to use JSON string instead of dict for Chat Completions API compatibility

Description

Related Issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

Testing

• 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 tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

Additional Notes

[github-actions]

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