feat: support asset caching for extension zip

[Soner (shyim)]

What

Adds an enable_asset_caching opt-in to build.zip.assets in .shopware-extension.yml, so extension zip reuses the existing asset cache machinery — mirroring the asset_caching setting already available for project ci.

build:
  zip:
    assets:
      enabled: true
      enable_asset_caching: true

Why

The cache layer is environment-agnostic (GitHub Actions cache in CI, disk cache otherwise) and already runs inside BuildAssetsForExtensions (restore + store). But the zip path never set EnableAssetCaching, so caching — including the already-configurable additional_caches — was silently inert during extension zip. Only project ci ever turned it on.

How

• internal/extension/config.go — add EnableAssetCaching to ConfigBuildZipAssets (yaml: enable_asset_caching).
• cmd/extension/extension_zip.go — wire it into the AssetBuildConfig passed to BuildAssetsForExtensions.
• internal/extension/config_schema.json — add the matching schema property (hand-maintained, embedded).
• internal/extension/asset_cache_test.go — tests for store/restore round-trip, disabled-is-no-op, and source-change busts the cache key.

Behavior / compatibility

Off by default (zero value false). Existing extensions are unaffected unless they explicitly opt in. Extensions that already configured additional_caches will only start caching once they set enable_asset_caching: true.

Notes (deliberately out of scope)

• The cache key (sw-cli-<shopwareVersion>-<contentHash>) does not include the CLI/esbuild version — a CLI upgrade alone won't bust the cache. Same as the existing project ci path, so not a regression; can be added separately if desired.
• The opt-in is a global flag on the zip path (zip builds one extension at a time). Making a per-extension flag also apply under project ci would require moving the boolean into the per-source entry — not done here.

Verification

• go build ./..., go vet ./internal/extension/... ./cmd/extension/... — clean
• go test ./internal/extension/ — passes (new tests included)
• gofmt — clean

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[chatgpt-codex-connector]

Use stable paths for zip asset cache keys

When extension zip runs with enable_asset_caching: true, the sources passed below come from tempExt under a fresh os.MkdirTemp directory for every invocation, but GetContentHash includes the collected absolute file paths in the hash. That means the same extension content gets a different cache key on each zip build, so restore always misses and the new opt-in cache only stores redundant entries instead of speeding up subsequent extension zip runs.

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