fix(server): preserve prompt icons in list responses

[he-yufeng]

Summary

Fixes #2054.

registerPrompt() accepted prompt metadata that includes icons, but the high-level server dropped that field before prompts/list serialized registered prompts. This keeps the icons on the registered prompt and includes them in list responses.

Test plan

• pnpm --filter @modelcontextprotocol/server test -- mcp.compat.test.ts
• pnpm --filter @modelcontextprotocol/server typecheck
• pnpm --filter @modelcontextprotocol/server lint
• pnpm --filter @modelcontextprotocol/server build
• pre-push hook: pnpm -r typecheck, pnpm -r build, pnpm sync:snippets --check, pnpm -r lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 17b4361

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/server	Patch

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commit: 17b4361