* refactor: Simplify content formatting in MCP service and parser
- Consolidated content handling in `formatToolContent` to return a plain-text string instead of an array for all providers, enhancing clarity and consistency.
- Removed unnecessary checks for content array providers, streamlining the logic for handling text and image artifacts.
- Updated related tests to reflect changes in expected output format, ensuring comprehensive coverage for the new implementation.
* fix: Return empty string for image-only tool responses instead of '(No response)'
When artifacts exist (images/UI resources) but no text content is present,
return an empty string rather than the misleading '(No response)' fallback.
Adds missing test assertions for image-only content and standardizes
length checks to explicit `> 0` comparisons.
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