fix: compatible with emoji/object icons in plugin card icon resolver

[IthacaDream]

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Summary

Fixes #33731

This PR updates the plugin card icon resolver to handle non-string (emoji/object) icons in addition to URL strings, enabling plugin cards to render AppIcon-style icons consistently.

Changes:

• Expanded getPluginCardIconUrl parameter type to accept { content: string; background: string }.
• Added an early return to pass object icons through without URL transformation.

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

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