[StaticWebAssets] Support disabling the task result caching, do not write the cache file if up to date or no assets

[javiercn]

• Makes sure to only write the file when it's out of date
• Enables a way to disable caching.

Fixes #48985

[dotnet-policy-service]

Thanks for your PR, @@javiercn.
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[Copilot]

Pull Request Overview

This PR adds the ability to disable the static web assets cache and prevents writing the cache file when it’s already up-to-date or when there are no assets to cache.

• WriteCacheManifest now only writes if the cache is out of date and there are input hashes.
• Introduces StaticWebAssetsCacheDefineStaticWebAssetsEnabled (default: true) to toggle caching and conditionally set cache path properties in several targets.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/StaticWebAssetsSdk/Tasks/DefineStaticWebAssets.Cache.cs	Guard WriteCacheManifest to skip file writes when up-to-date or no inputs
src/StaticWebAssetsSdk/Targets/Microsoft.NET.Sdk.StaticWebAssets.targets	Add StaticWebAssetsCacheDefineStaticWebAssetsEnabled property and apply it to project cache path
src/StaticWebAssetsSdk/Targets/Microsoft.NET.Sdk.StaticWebAssets.JSModules.targets	Apply the cache-enable condition to JS modules cache paths
src/StaticWebAssetsSdk/Targets/Microsoft.NET.Sdk.StaticWebAssets.Compression.targets	Apply the cache-enable condition to compressed assets cache path