Compute platform beta status when using @Available

Changes:
- Sets `AvailabilityRenderItem.isBeta` when `AvailabilityRenderItem.init?(_ :current:)` is called
- Makes `AvailabilityRenderItem.isBeta`, `AvailabilityRenderItem. introduced` and `AvailabilityRenderItem.name`  into `let` properties as they are integral to availability rendering.

Unifies the behavior in `AvailabilityRenderItem` so that we always compute whether a platform is in beta, regardless of whether the availability information comes from the symbol graph or comes from the `@Available` directive.

This fixes an issue where sample code articles defined with `@Available` directives would never display a beta badge regardless of whether the platform was configured to be a beta platform.

Additionally, makes `isBeta` a `let` property to avoid any future bugs, as the code will now no longer compile unless this property is set in an initializer.

Resolves rdar://129355087.

Sources/SwiftDocC/Model/Rendering/Symbol/AvailabilityRenderMetadataItem.swift

@@ -55,10 +55,10 @@ extension VersionTriplet {
 /// Availability information of a symbol on a specific platform.
 public struct AvailabilityRenderItem: Codable, Hashable, Equatable {
     /// The name of the platform on which the symbol is available.
-    public var name: String?
+    public let name: String?
 
     /// The version of the platform SDK introducing the symbol.
-    public var introduced: String?
+    public let introduced: String?
 
     /// The version of the platform SDK deprecating the symbol.
     public var deprecated: String?
@@ -82,7 +82,7 @@ public struct AvailabilityRenderItem: Codable, Hashable, Equatable {
     public var unconditionallyUnavailable: Bool?
 
     /// If `true`, the symbol is introduced in a beta version of this platform.
-    public var isBeta: Bool?
+    public let isBeta: Bool?
 
     private enum CodingKeys: String, CodingKey {
         case name, introducedAt, deprecatedAt, obsoletedAt, message, renamed, deprecated, unavailable
@@ -131,12 +131,7 @@ public struct AvailabilityRenderItem: Codable, Hashable, Equatable {
         renamed = availability.renamed
         unconditionallyUnavailable = availability.isUnconditionallyUnavailable
         unconditionallyDeprecated = availability.isUnconditionallyDeprecated
-
-        if let introducedVersion, let current, current.beta, introducedVersion == current.version {
-            isBeta = true
-        } else {
-            isBeta = false
-        }
+        isBeta = AvailabilityRenderItem.isBeta(introduced: introducedVersion, current: current)
     }
 
     init?(_ availability: Metadata.Availability, current: PlatformVersion?) {
@@ -146,6 +141,15 @@ public struct AvailabilityRenderItem: Codable, Hashable, Equatable {
         let platformName = PlatformName(metadataPlatform: availability.platform)
         name = platformName?.displayName
         introduced = availability.introduced.stringRepresentation(precisionUpToNonsignificant: .minor)
+        isBeta = AvailabilityRenderItem.isBeta(introduced: availability.introduced, current: current)
+    }
+
+    static func isBeta(introduced: VersionTriplet?, current: PlatformVersion?) -> Bool {
+        guard let introduced, let current, current.beta, introduced == current.version else {
+            return false
+        }
+
+        return true
     }
 
     /// Creates a new item with the given platform name and version string.