fix(cloudflare): add namespace filter to astro-frontmatter-scan esbuild plugin

[barry3406]

Fixes #16203

Problem

The astro-frontmatter-scan esbuild plugin registers its onLoad handler without a namespace filter. In esbuild, an unfiltered onLoad matches all namespaces, including html. When a .ts file default-imports an .astro component, Vite's dep scanner resolves the .astro file into the html namespace. The plugin intercepts it and returns only the extracted frontmatter — which has no export default — causing:

[ERROR] No matching export in "html:/path/to/Component.astro" for import "default"

Fix

Add namespace: 'file' to the onLoad call so the plugin only handles .astro files resolved from the filesystem. Files in the html namespace now fall through to Vite's built-in htmlTypeOnLoadCallback, which already appends export default {} for non-.vue files.

Test plan

• Verified against the reproduction repo from the issue: error no longer occurs
• file namespace behavior is unchanged — same handler, same logic, only the namespace scope is narrowed

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