feat(esbuild): content-addressable asset building & Vite manifest generation

[shyim]

User story

As a developer building extension assets with shopware-cli,
I would like content-addressable filenames (hashed JS & CSS) registered in Vite manifest for Shopware 6.7+ while preserving unhashed filenames for pre-6.7 compatibility,
so that browser caches properly reset when assets are updated without breaking older Shopware versions.

Acceptance criteria

• Content-addressed filenames (e.g. js/<name>-<hash>.js and css/<name>-<hash>.css) are emitted during esbuild asset compilation using an 8-character content hash (xxhash).
• .vite/manifest.json and .vite/entrypoints.json are updated to point to the content-addressed JS & CSS asset filenames.
• Unhashed asset files (js/<name>.js and css/<name>.css) are preserved in the output directory for backward compatibility with Shopware < 6.7.
• Unit tests are added/updated to verify content-addressable filename generation and Vite manifest content.

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.