fix(astro): resolve middleware export types

[itosa-kazu]

Fixes #21413.

Summary

• Point the @sentry/astro/middleware export types condition at the emitted index.d.ts declaration file.
• Avoid exposing the Astro-version-specific MiddlewareResponseHandler alias from the middleware subpath declaration, so the published declaration remains readable for supported Astro versions where that alias is absent.
• Preserve the existing runtime behavior by continuing to delegate to handleRequest().

Validation

• git diff --check
• JSON assertion that exports[./middleware].types is ./build/types/integration/middleware/index.d.ts
• Temporary consumer project with @sentry/astro@10.57.0, astro@5.16.6, and typescript@5.9.3: original package reproduces TS7016; patched package metadata plus the expected declaration shape passes npx tsc --noEmit; runtime import still exports onRequest.

A direct package type-check in this sparse checkout was not runnable because the checkout/install did not include linked Sentry workspace packages such as @sentry/core, @sentry/browser, and @sentry/node.