[dart_mcp_server] Update link of "Add to Cursor" button

[parlough]

Fixes the "Add to Cursor" button.

Mirrors dart-lang/site-www#6801 by updating to the new URL scheme documented in at https://docs.cursor.com/en/tools/developers#mcp-servers.

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[jakemac53]

thanks!

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autosubmit label was removed for dart-lang/ai/265, because This PR has not met approval requirements for merging. The PR author is not a member of dart-team and needs 1 more review(s) in order to merge this PR.

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[auto-submit]

autosubmit label was removed for dart-lang/ai/265, because - The status or check suite build (master, windows-latest) has failed. Please fix the issues identified (or deflake) before re-applying this label.

[jakemac53]

landing this on red, its just flakes