fix: legend marker shapes don't match chart when only shape channel is encoded (#7304)

* fix: legend shape inconsistency when only shape mapping is used (issue #7286)

Agent-Logs-Url: https://github.com/antvis/G2/sessions/f53d6e75-b912-4210-a7be-aac3297fd1d3

* feat: add snapshot for shape scale

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: moayuisuda <37040897+moayuisuda@users.noreply.github.com>
Co-authored-by: 林舸 <anhaohui.ahh@antgroup.com>

Author: 3 people