fix(cloudflare): discover server-side dependencies in .astro frontmatter

[matthewp]

Changes

• Adds an esbuild plugin that extracts frontmatter from .astro files during Vite's dependency optimization scan phase
• This allows server-side imports in .astro frontmatter to be discovered ahead of time, avoiding the "new dependencies optimized" message and page reload during dev

Previously, Vite's scanner only looked for <script> tags in .astro files, missing the frontmatter section where server-side imports live. This caused npm dependencies like ms to be discovered lazily on first request.

This is only done in the Cloudflare adapter because dependency optimization is not needed in Node.js where these deps are external.

Testing

Added a new test fixture ssr-deps with a test that:

1. Starts a dev server
2. Clears the Vite cache
3. Makes a request to a page that imports a third-party dependency (ms) in frontmatter
4. Verifies no "new dependencies optimized" log message appears

Docs

N/A, bug fix

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Latest commit: 18ade2d

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