Suppress structuredContent when visible Content is present (#240)

## Summary

Refines the human-readable output formatting for tool call results to
avoid redundant information. When a tool result contains both `content`
(with visible text blocks) and `structuredContent`, the structured
content is now suppressed since it duplicates information already
conveyed by the content blocks. The full payload remains available via
`--json` mode.

## Changes

- **Output logic**: Modified `formatCallToolResultHuman()` to only
display `structuredContent` when there is no visible `content`. This
reduces noise in human-readable output while preserving all information
in JSON mode.

- **Test updates**: Updated and added test cases to reflect the new
behavior:
  - `structuredContent` is shown only when `content` is empty
- `structuredContent` is suppressed when visible content blocks are
present
- Verified correct section ordering: Content → Metadata (when content
present) or Structured content → Metadata (when content empty)

- **Documentation**: Updated JSDoc comments in `output.ts` to clarify
that `structuredContent` is shown only when there is no visible Content,
and that the full payload is available via `--json`.

## Implementation Details

- Introduced `visibleContent` variable to track content blocks after
filtering out duplicates, making the logic clearer and more maintainable
- The suppression applies consistently: any visible content (even a
single text block) causes `structuredContent` to be hidden
- This aligns with the design principle of avoiding unnecessary verbose
output for LLM consumers while maintaining full transparency in
structured output modes

https://claude.ai/code/session_01V9qveFdBMjXQAb1wcZnE9b

Co-authored-by: Claude <noreply@anthropic.com>