fix: resolve snippets in magic move blocks

[cyphercodes]

Fixes #2586.

Summary

• Resolve snippet imports inside md magic-move blocks before validating inner code blocks.
• Reuse the existing snippet import resolver so imported files, regions, languages, metadata, and watch files stay consistent.
• Add a regression test covering magic move with snippet inclusions.

Tests

• corepack pnpm vitest run packages/slidev/node/syntax/codeblock/magic-move.test.ts
• corepack pnpm vitest run packages/slidev/node/syntax/snippet.test.ts packages/slidev/node/syntax/integration.test.ts packages/slidev/node/syntax/codeblock/magic-move.test.ts --testTimeout 20000
• corepack pnpm eslint packages/slidev/node/syntax/snippet.ts packages/slidev/node/syntax/codeblock/magic-move.ts packages/slidev/node/syntax/codeblock/magic-move.test.ts
• corepack pnpm --filter @slidev/cli... build
• git diff --check

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commit: c4e73b0