[StaticWebAssets] Fix Vary header in Static Web Assets compression negotiation to use Accept-Encoding (#50151)

The ApplyCompressionNegotiation task was incorrectly setting the Vary header to "Content-Encoding" when serving compressed static web assets. According to HTTP specifications, the Vary header should indicate which request headers the server considers when determining the response, not which response headers are set.

When serving compressed content, the server should set Vary: Accept-Encoding to indicate that the response varies based on the client's Accept-Encoding request header (which tells the server what compression formats the client supports).

Author: Copilot

src/StaticWebAssetsSdk/Tasks/ApplyCompressionNegotiation.cs

@@ -191,7 +191,7 @@ private static StaticWebAssetEndpointResponseHeader[] CreateCompressionHeaders(S
             new()
             {
                 Name = "Vary",
-                Value = "Content-Encoding"
+                Value = "Accept-Encoding"
             }
         ];