Document supported issue URLs for save_note

What does this MR do and why?

The save_note URL parser already routes standard issue URLs through the shared work item resolver, but the URL documentation and unsupported-URL error did not reflect all accepted formats. This updates those descriptions and adds issue and epic URL resolution coverage, while the existing MR URL spec covers merge requests.

References

#595158

Screenshots or screen recordings

Not applicable; this change does not affect the UI.

Before	After
N/A	N/A

How to set up and validate locally

1. Run bin/rspec spec/services/mcp/tools/notes/save_note_tool_spec.rb spec/services/mcp/tools/notes/save_note_service_spec.rb.
2. Run bundle exec rubocop app/services/mcp/tools/notes/save_note_tool.rb app/services/mcp/tools/notes/save_note_service.rb spec/services/mcp/tools/notes/save_note_tool_spec.rb spec/services/mcp/tools/notes/save_note_service_spec.rb.
3. Run git diff --check.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.