fix: improve error message for missing exports in file routes

[bartlomieju]

Summary

• Improves the unhelpful error message when a file-system route file has no recognized exports
• Now shows specific hints based on file type (middleware vs route) with code examples

Before:

Error: Expected a route, middleware, layout or error template, but couldn't find relevant exports in: routes/_middleware.ts

After (middleware):

Error: Could not find relevant exports in: routes/_middleware.ts

Middleware files must have a default export (function or array of functions).

  Example:
    export default define.middleware(async (ctx) => {
      return await ctx.next();
    });

After (route):

Error: Could not find relevant exports in: routes/api/users.ts

Route files must export a default component, a "handler" or "handlers" export, or a "config" export.

  Example:
    export const handler = define.handlers({ GET(ctx) { ... } });
    export default define.page((props) => <h1>Hello</h1>);

Test plan

• deno test -A packages/fresh/src/fs_routes_test.tsx — 55 passed

Closes #3652

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